Over the past years, camcorders have excelled in capturing video, but the quality of the audio remains lagging.  Adding an external microphone is a must, but will only get you so far.  Quiet voices will barely rise above the noise floor, while it feels like the videographer yelling in your ear.

This article describes a three stroke approach to improve sound from camcorders.  The first step is to reduce the noise.  In the following steps the dialog loudness is set to a standard level and the dynamic range is reduced.  The last two steps are an integral part of AC-3 (A/52, Dolby Digital) but can also be applied to other streams such as Advanced Audio Codec (AAC) as shown in the last section.

Noise reduction

First things first.  Start with a good external microphone and a wind muff.  Personally, I use a Canon 5.1 microphone because it is small and mounts on the hot shoe.  The specs are not that impressive, but even an ideal microphone will pick up a significant amount of ambient noise.

The noise level is defined as the loudness perceived by the human ear.  The loudness is commonly measured using a frequency filter that mimics the human hearing and then measuring the energy as root-mean-square (LAeq).  Note that for a sine wave the RMS level is √2 × peak level.  Another popular measure is the K-scale for which there are several plugins such as mzuther's, DPMP, and meterplugs.

Personally, I take a shortcut by not applying the filter and simply measuring the RMS level of noise samples.  For this, I use the stats function from Sound Exchange.  In practice this appears to be good enough.  My typical indoor measurements are shown in the table below.  The values in the table are expressed at decibels below digital full-scale.

With the quiet dialog level this close to the noise floor, we can't amplify this dialog without also significantly amplifying the noise.  As a first step we lower the noise floor in post using an audio editor such as Adobe Sound Booth.  This usually reduce the noise by about 8 dB without causing significant distortion.

Dialog Normalization

The volume dial on AC-3 decoders controls the loudness of a normal dialog.  The listener is thus able to reliably set the volume level of the dialogue no matter what program is playing.  This does however require the audio to indicate the normal dialog level at which it is recorded.  In AC-3 this is accomplished using the metadata tag "dialnorm" as described in the AC-3 specification §5.4.2.8 and §7.6.  The decoder uses this parameter to automatically adjust the volume so that the dialog is always played at the same loudness.

Dialog loudness expresses how an average person perceives the volume of a dialog [DD1; DD2].  The film industry has long standardized the normal dialog level at -31 dBFS.  The corresponding Sound Pressure Level (SPL) for most movie theaters is 85 dB.

At home we have a volume dial that selects the SPL for normal dialog.  Assuming the volume dial is set to a sound pressure level of 67 dB for normal dialog.  When this decoder plays a stream with a normal dialog level of -31 dBFS, it will adjust the amplifier gain so that 0 dBFS ≡ 98 dB, what causes the normal dialog to reproduce at -31 dBFS ≡ 67 dB.  If the listener switches to another program with a normal dialog at -25 dBFS, the amplifier gain will be reduced so that 0 dBFS ≡ 92 dB, and the normal dialog stays at a 67 dB.

Dynamic Range Compression

The sound from camcorders often has an undesirable large dynamic range.  On one end, there may be people whispering in the distance, while on the other end we may have the videographer holding the camcorder.  The difference between these loudness levels in called dynamic range.  We can reduce the dynamic range by both amplifying the quiet sounds and attenuating the loud sounds.  Note that to successfully boost the quiet sounds, it is essential that they are well above the noise floor.

With AC-3 we can adjust the gain using the metadata tag "dynrng" as described in §7.7 of the AC-3 specification.   The AC-3 encoder generates these tags based on the loudness combined with a compression profile.  All standard profiles have a null-band that is centered around the normal dialog level.  Loudness levels within this null-band are left intact.  For the film compression profile at a normal dialog level of -31 dBFS, the transfer function can be visualized as shown below.

As shown in the graph above, signals from -33.5 to -28.5 dBFS stay the same.  The 12 dB below is boosted, while the 10 dB above is attenuated as also shown in the table below.

This film at dialnorm -31dBFS profile is a good fit for my camcorder recordings.  The quiet dialog is more or less in the middle of the boost range, the normal dialog is at the normal dialog level, while the loud dialog is in the early cut range.

The table above shows that the soft dialog is amplified by 4 dB, but at the cost of raising the noise level 2 dB.  If we had a constant background noise level, we could consider alter the transfer function so that it attenuate all signals below -45.5 dBFS.

Let's do it

As shown above, applying dialog normalization and dynamic range is straightforward using the metadata in AC-3.  For other audio streams, we will need to alter the samples to get the same effect.

AC-3

For AC-3, we only need to extract the audio stream using FFmpeg and compress it using Aften.

ffmpeg -y -vn -i in.avi out.wav
aften -v 0 -dnorm 31 -dynrng 1 out.wav out.ac3

AAC

The Advanced Audio Codec (AAC) doesn't support the metadata for dialog normalization or dynamic range compression.  Instead, we modify the samples directly using SoX as shown below.  Note that in this particular example it also applies a 13 dB overall gain.  The values printed in bold represent the compression curve.  The resulting stream is then compressed using Nero's AAC encoder.

ffmpeg -y -vn -i in.avi tmp.wav
set CURVE=-90.0,-84.0,-45.5,-39.5,-33.5,-33.5,-28.5,-28.5,-18.5,-23.5,0.0,-22.6
sox tmp.wav out.wav compand 0.1,3.0 %CURVE% 13.0 -90 1.6 stats
neroaacenc -lc -br 226000 -if out.wav -of out.aac

Those who like to experiment themselves find the transfer curves for other profiles at various dialog levels.

• film light

  -31:   -90.0,-84.0, -53.0,-47.0, -41.0,-41.0, -21.0,-21.0, -11.0,-16.0, 0.0,-15.5
  -28.5: -90.0,-84.0, -50.5,-44.5, -38.5,-38.5, -18.5,-18.5,  -8.5,-13.5, 0.0,-13.1
  -27:   -90.0,-84.0, -49.0,-43.0, -37.0,-37.0, -17.0,-17.0,  -7.0,-12.0, 0.0,-11.7

• film

  -31:   -90.0,-84.0, -45.5,-39.5, -33.5,-33.5, -28.5,-28.5, -18.5,-23.5, 0.0,-22.6
  -28.5: -90.0,-84.0, -43.0,-37.0, -31.0,-31.0, -26.0,-26.0, -16.0,-21.0, 0.0,-20.2
  -27:   -90.0,-84.0, -41.5,-35.5, -29.5,-29.5, -24.5,-24.5, -14.5,-19.5, 0.0,-18.8

• speech

  -31:   -90.0,-74.8, -52.5,-37.3, -33.5,-33.5, -28.5,-28.5, -18.5,-23.5, 0.0,-22.6
  -28.5: -90.0,-74.8, -50.0,-34.8, -31.0,-31.0, -26.0,-26.0, -16.0,-21.0, 0.0,-20.2
  -27:   -90.0,-74.8, -48.5,-33.3, -29.5,-29.5, -24.5,-24.5, -14.5,-19.5, 0.0,-18.8

• music light

  -31:   -90.0,-78.0, -65.0,-53.0, -41.0,-41.0, -21.0,-21.0, -21.0,-21.0, 0.0,-20.0
  -28.5: -90.0,-78.0, -62.5,-50.5, -38.5,-38.5, -18.5,-18.5, -18.5,-18.5, 0.0,-17.6
  -27:   -90.0,-78.0, -61.0,-49.0, -37.0,-37.0, -17.0,-17.0, -17.0,-17.0, 0.0,-16.2

• music

  -31:   -90.0,-78.0, -57.5,-45.5, -33.5,-33.5, -28.5,-28.5, -18.5,-23.5, 0.0,-22.6
  -28.5: -90.0,-78.0, -55.0,-43.0, -31.0,-31.0, -26.0,-26.0, -16.0,-21.0, 0.0,-20.2
  -27:   -90.0,-78.0, -53.5,-41.5, -29.5,-29.5, -24.5,-24.5, -14.5,-19.5, 0.0,-18.8

For more information

• Properly encoding AC-3
• Measuring LAeq
• Callibrate SPL

Work Flow

• Drag and drop blocks from the palette onto the work space.  Each block performs an unique function such as turning the motors, or measuring a distance.
• Use the tabs on the bottom of the palette to switch between sets of blocks.
• To save your work as a package use Tools > Create Pack and Go.
• As always, keep it simple.  Divide up the problem in small steps, and test each step before combining them.

• middle-button, compiles the program, downloads and runs it to  the NXT
• top-right button, compiles selected blocks, downloads and runs it to  the NXT
• bottom-left button, compiles the program and downloads it to  the NXT
• bottom-right button, stops the program on the NXT.
• top-left button, manages the NXT memory space, click the top-left button; click on the bar graph on the left to select a memory.  This will also let you transfer files.

I/O blocks (equiv. to library functions in C)

• Motor action
  • Motor block
    • Controls a single motor
    • Automatically corrects for rotation errors.
    • Parameters:
      • action, constant or ramp up/down until target is met.
      • power, <10 will not move; >75 moves out of control.
      • control, automatically adjust the power to correct for resistance or slipping.
      • duration, unlimited = switches the motor on and continues with the next block on the Sequence beam.
      • wait, when unchecked, it will move on to the next block on the Sequence beam while the motor meanwhile completes its action.
  • Move block
    • Controls two motors, synchronizing the B and C motors
    • Automatically corrects for rotation errors.
    • Parameters (also see the Motor block):
      • steering, middle = straight; far left/right = spin in place (motors turn opposite directions).
      • next action, brake = provides accurate travel distance and then hold the motor in place; coast = stop the power to the motor so it rolls to a stop (Because the motor coasts to a stop, it will move a little further than the Duration setting. When a following Move or Motor block runs, it will automatically adjust for the rotation error.)
  • Reset
    • Resets the accumulated rotation error, causing the next Motor or Move block to disregard prior rotation errors.  The Reset block should precede the first Motor or Move block in a program.
• Other action blocks
  • Sound
    • Plays a sound file or a single tone
    • Parameters:
      • wait, when unchecked, the next block on the Sequence beam will start while the sound is playing.
  • Display
    • Writes a text string or shapes to the NXT display
  • Send message
    • Transmits a wireless message to another NXT assuming it has been paired before.  Each NXT has 10 mailboxes each 5 messages deep.  Head-drop when mailbox is full.
  • Color lamp
    • Turns a color lamp on or off.

• Sensor blocks
  • Ultrasonic Sensor block
    • Measures the distance to an object and compares it to a criterion.
    • Outputs:
      • yes/no, true if criterion is met, false otherwise
      • distance, scaled value from sensor [0..255 cm | 0..100 inches].
  • Touch Sensor block
    • Reads the touch sensor's state and compares it to a criterion (pressed, released, bumped). Note that a program may not be fast enough to detect the "bumped" state.
    • Outputs:
      • yes/no, true if criterion is met, false otherwise
      • raw value, unscaled value from sensor [0..1024]
      • logic number, 0=released; 1=pressed.
  • NXT Buttons block
    • Reads a NXT button state and compares it to a criterion (pressed, released, bumped). Note that your program may not be fast enough to detect a condition as "bumped".
    • Output:
      • yes/no, true if criterion is met, false otherwise.
  • Color Sensor block
    • Measures color or light intensity and compares it to a criterion.  When the parameter "light" is checked it generates its own light and measure the reflected light.
    • Outputs:
      • yes/no, true if criterion is met, false otherwise
      • detected color, light intensity [0..100] or detected color (1=black; 2=blue; 3=green; 4=yellow; 5=red; 6=white).
  • Rotation sensor block
    • Measures the number of degrees that the motor turned and compares it to a criterion.  The same block can also be used to reset the rotation counter.
    • Output:
      • yes/no, true if criterion is met, false otherwise
      • direction, true=forwards; false=backwards
      • degrees, scaled value from sensor [0..2147483647].
• Other blocks
  • Timer block
    • Reads one of the tree timers and compares it against a criterion.  Time is specified in seconds on the configuration panel, but in milliseconds when using input data wires.  The block can also be used to reset a timer.
    • Output:
      • yes/no, true if criterion is met, false otherwise
  • Receive Message block
    • Receives a Bluetooth message and compare it against a criterion.
    • Outputs:
      • yes/no, true if criterion is met, false otherwise
      • message received, true when a message has been received, false otherwise
      • {text|number|logic} out, message data.
  • Random block
    • Generates a random number within a range.  Can be used to make the robot a little unpredictable and thus more interesting.
    • Output:
      • number, random value between specified lower and upper bounds.

• Numbers
  • Math Block
    • Performs basic math operations (addition, subtraction, multiplication, division, square root, absolute value).
    • Output:
      • result, result of the operation [-2147483648 .. 2147483647]
  • Logic Block
    • Performs Boolean operations (AND, OR, XOR or NOT)
    • Output
      • result, result of the operation [true|false]
  • Compare Block
    • Performs less than (<), greater than (>) or equal (=) operations
    • Output
      • result, result of the operation [true|false]
  • Range Block
    • Determines if a value is within a range of numbers
    • Output
      • yes/no, true of the value is within range, false otherwise
• Text strings
  • Number to Text block
    • Converts a number to a text string
    • Output
      • text, text representation of number [string]
  • Text block
    • Concatenates a maximum of three text strings
    • Output
      • text, concatenation of text strings [string]

Program Flow

• Conditionals
  • Switch block (equiv. to switch())
    • Checks for a condition, and runs blocks on the sequence beam corresponding to that condition.
    • More then two conditions are allowed.
    • The '*' button selects the default condition.
    • Floating point inputs are rounded to the nearest integer before comparing them.
    • Comments should be left outside a switch block.
• Wait
  • Wait block
    • Busy-loop to poll an input until the specified condition is met.
• Loops
  • Loop block (equiv. to do while())
    • Repeats a set of blocks until the end condition is met.
    • The loop count output is updated at the start of the last run through the loop, it will be one fewer than the total number of times the loop repeats.

My blocks (equiv. to user functions)

• Usage
  • Create, select the blocks; "Edit > Make a new My Block" from the menu
  • Open, double-click the My Block
  • Use, select from the Custom Palette (using the tab at the bottom of the Palettes)
  • Close, "File > Close" from the menu
  • Organize, "Edit > Manage Custom Palette" from the menu; then use the file system to organize.
• Creating My blocks that have a data hub
  • Use compare, text or math blocks to supply input values to the blocks that will become a new My block.  These additional blocks are only needed to supply the data wire, and will not become part of the My block.
  • Select the blocks for the new My Block and call "Edit > Make a new My Block" from the menu.  All the data wires will remain connected. The value of the wire will also become configurable using the Configuration Panel.
  • Note that once a My block is made, you can change the name of data plugs, but you can not add more plugs.  You will have to recreate the My block instead.
• Appears broken if
  • two data plugs have the same name, or
  • there is problem with the data wire connections, or
  • the source file for the block has been moved or renamed in the computer's file system.
• Note
  • If you absolutely have to copy a My block, then use the file system.  Don't use "save as" from the menu.
  • When moving a program to another computer, also move the My Blocks or better yet create a package that contains all the related files ("Tools > Create Pack and Go")

Data

• Scope
  • Local to the blocks that it connects to the wire.
• Usage
  • draw a wire between blocks
    • expand the data hubs (click on the tab at the bottom of a block).
    • the plugs on the left are inputs; the plugs on the right are outputs.
    • click on an output terminal
    • click on the input terminal that you want to connect.
  • draw a wire into the boundary of a Switch block
    • enable tabbed view (disable flat view)
    • connect a wire to the input
    • switch back to flat view
  • draw wires from within the boundary of a Switch block
    • enable tabbed view
    • select the "true" condition and connect a wire
    • select the "false" condition and connect a wire from the output to the edge of the block where the existing wire crosses it.
    • switch back to flat view
  • delete a wire
    • click the data plug at the right of the wire.
• Note
  • Use variables instead of passing data wires in/out of a Loop or Switch block.
  • Occasionally, a wire may be routed behind another block, making it appear as if its terminals are connected.
  • A dashed grey wire indicates a problem, such as
    • two plugs have different data types (Boolean, integer, float, string)
    • the output of a pass-through plug is not used without the input side connected
    • multiple inputs are connected to one output plug.
    • an input is connected to an output that appears later on the sequence beam.

• Scope
  • Local to the program (and its My blocks) that defines it.
• Usage
  • create a variable, "Edit > Define Variables" from the menu.
  • use a variable, insert a Variable block.
  • delete a variable, first remove any Variable blocks that use it.
• Notes
  • A program shares its variables with its My blocks if they have the same name and data type.
  • Perhaps not the best coding practice, but variables can be used when a My Block needs to remember a value.

• Scope
  • Global to all programs.
• Usage
  • create, "Edit > Define Constants" from the menu.  Specify the action "choose from list".
  • use, insert a Constant block.
  • change, save any open programs that use that constant.
  • delete, "Edit > Define constants" from the menu.  Note that any Constant block that uses that constant will change to Custom.
• Note
  • When working with a program that uses custom constants, note that:
    • changing the value in one Constant block won’t affect the other programs.
    • creating a constant using the Edit Constant dialog will change all "Custom" constants with the same name.

• Scope
  • Global to all programs running on a NXT.  Remain available after the program ends.
• Usage
  • create, insert a File Access block; select create as the action.  A list of files already on the NXT will appear (assuming it is connected using either USB or BT).
  • write, insert a File Access block; select write as the action to appends to a file.
  • read, insert a File Access block; select read as the action.
  • delete, insert a File Access block; select delete as the action.
  • close, insert a File Access block; select close.  Remember to close a file after writing to it.  Otherwise you can not delete or read from it.
• Note
  • A program can write to or read from up to four files at once.
  • The Initial File Size data plug that allows you to specify how much memory to allocate for the file. If the file grows past the initial size, the NXT will move the file to larger memory space, thereby introducing a slight delay.

Sequence Beams (=threads).

Multitasking means doing more than one thing at a time. The sequence beam can fork, allowing your robot to do more than one thing at a time.  But first, a short word about execution order: Execution order

• In general, a block will finish what it’s doing before the next block on the Sequence Beam runs.  Exceptions are when "wait for completion" is unchecked, or a Move/Motor block is set to run indefinitely.
• A block starts once all input values on the data wires are available.
• A Loop/Switch block starts only once all the input values are available on data wires that enter the block (even though they might not be immediately used)
• A data wire that starts inside a Loop/Switch block and connects to a block outside will only have a value after the Loop/Switch block finishes.
• Only one copy of a particular My Block can run at the same time (non-re-entrant)
• A program ends once it reaches the end of all the Sequence beams.

To add a task

• Expand vertical space:
  • Add a Display block (that has a large data hub) at the end of the Loop block.
  • Open the data hub to create space within the loop.
  • Add a small comment to the bottom-left side of the Loop block to keep it open after the Display block is removed.
  • Delete the display block.
• Add a new Sequence beam:
  • Add a new block under the existing Sequence beam.
  • Shift-click the existing Sequence Beam, and connect it to the new block.
• To expand horizontal space:
  • Select the outer Loop block.
  • Click-and-hold the mouse on the Sequence Beam between the top-right block and the edge of the Loop block.  Slowly drag the mouse to the right to widen the loop block.

Note

• Especially when working with multiple Sequence beams, allow the NXT-G some time to catch up with your editing.
• Do not access the same sensor or motor from different Sequence beams.
• Blocks on different sequence beams can communicate using variables, not pure data wires.

Debugging

Often debugging a program proves to be more challenging than writing it.  The NXT-G environment offers the following debug approaches:

Wireshark is a open-source protocol analyzer that can visualize the frames passing through the router. This analyzer is available on many platforms, but can not run directly on the router.

using tcpdump

While one could run tcpdump directly on the router, interpreting its output takes a bit of work. Another option run tcpdump on the router and pipe its output to wireshark on a Linux host. Details can be found in Remote Packet Capture. An excerpt.  To capture traffic on bridge br0, use the following commands on a Linux host.

mkfifo /tmp/$PPID
ssh rtr "tcpdump -i br0 -w - 'not ((src host rtr and src port 22) or (dst host rtr and dst port 22))'" > /tmp/$PPID. &
wireshark -k -i /tmp/$PPID

using etherpuppet

Instead we forward all packets from an interface on the router to a Linux host, where they the protocol analyzer runs.

Prepare the router

cd /jffs
wget http://www.secdev.org/projects/etherpuppet/files/etherpuppet-mipsel
chmod 755 etherpuppet-mipsel

Prepare the Linux host

wget http://hg.secdev.org/etherpuppet/raw-files/top/etherpuppet.c
gcc -o etherpuppet etherpuppet.c
sudo yum install wireshark-gnome
sudo usermod -a -G wireshark $NAME

Instruct the router to forward all packets on the bridge that joins the LAN and wireless traffic (br0).

./etherpuppet-mipsel -i br0 -s 999 -C

Receive the packets on the Linux host and start the protocol analyzer.

sudo ./etherpuppet -m -c rtr2.vonk:999 &

sudo ifconfig puppet0 up
sudo wireshark # select interface puppet0

In Wireshark select the puppet0 interface.

• VLANs, allows wired LAN ports to be grouped and each group to be treated differently.
• multiple SSIDs, allows multiple wireless network names (SSID) and security settings.  The traffic over each SSID can be treated differently.

Before we start

• the web based user interface (WebGUI)
• networking concepts such as IP addresses, sub nets, routing, bridging and network interfaces.  Some of the more specific terminology used in this article is:
  • A broadcast domain, is a network segment, in which all nodes can communicate directly by layer-2 broadcasts.  Examples are a wired GigE network, or a wireless network.
  • A switch, handles frames at the data link layer (layer 2).
  • A router, handles packets at the network layer (layer 3).  A router forms a boundary between broadcast domains.
  • A bridge, connects two or more network segments into a single broadcast domain (layer 2).
  • A virtual LAN (VLAN), creates distinct broadcast domains within one ore more physical networks.  A VLAN is a switched network that is logically segmented by functions, project teams, or applications without regard to the physical location of users.

Inside the router

Hardware block diagram

• System-on-Chip BCM4718, provides the CPU, WiFi and USB.
• Switch Fabric BCM53115, provides  the WAN and four LAN ports

• the LAN port numbers on the case do not correspond to the port numbers on the switch;
• the WAN port connects to a special interface on the switch and can't be reassigned;
• port 8 connects the VLAN trunk from the switch to interface eth0 on the CPU.

Firmware mapping

nvram show | grep vlan.*ports | sort
vlan0ports=1 2 3 4 5*
vlan1ports=4 3 2 1 8*
vlan2ports=0 8

nvram show | grep port.*vlan | sort
port0vlans=2 18 19
port1vlans=1 18 19
port2vlans=1 18 19
port3vlans=1 18 19
port4vlans=1 18 19
port5vlans=1 2 16

nvram show | grep vlan.*hwname | sort
vlan0hwname=et0
vlan1hwname=et0
vlan2hwname=et0

Creating to separate networks

Create a virtual wireless network (wl0.1)

1. Wireless > Virtual Interfaces > Add.  Specify the basic wireless settings.  For the network configuration, choose bridged.  Click Save.
2. Wireless > Wireless Security > Virtual Interface wl0.1.  Specify the security settings.  Click Apply Settings, to save and apply the changes.

Create a virtual local area network (vlan3)

nvram set vlan3hwname=et0       # VLAN3 is enabled
nvram set vlan1ports="3 4 8*"   # assign LAN1/LAN2 to VLAN1
nvram set vlan3ports="1 2 8*"   # assign LAN3/LAN4 to VLAN3
nvram set port4vlans="1 18 19"  # LAN1 is part of VLAN1 (GUI only)
nvram set port3vlans="1 18 19"  # LAN2 is part of VLAN1 (GUI only)
nvram set port2vlans="3 18 19"  # LAN3 is part of VLAN3 (GUI only)
nvram set port1vlans="3 18 19"  # LAN4 is part of VLAN3 (GUI only)
nvram commit
reboot

Bridge vlan3 and wl0.1 (br1)

1. Bridge the vlan3 and wl0.1 interfaces together as br1
   • Setup > Networking > Bridging > Create Bridge > Add.  Enter br1 in the first field and click Apply Settings.
   • Setup > Networking > Bridging > Create Bridge > For br1 enter a private IP address and subnet mask.  E.g. 10.0.4.1 and 255.255.255.0.  Click Apply Settings.
   • Setup > Networking > Assign to bridge > Add.  Select br1, interface vlan3 (LAN3 and LAN4) and click Apply Settings.
   • Setup > Networking > Assign to bridge > Add.  Select br1, interface wl0.1 (the virtual wireless interface) and click Apply Settings.
2. Configure the DHCP server for br1
   • Setup > Networking > DHCPD > Multiple DHCP Server > Add.  Select br1 and fill in the first m maximum number of DHCP addresses and lease time in minutes.  E.g. 200, 50 and 1440.  Click Apply Settings.

Separate the networks

iptables -I FORWARD -i br0 -o br1 -m state --state NEW -j DROP
iptables -I FORWARD -i br1 -o br0 -m state --state NEW -j DROP

iptables -I INPUT -i br1 -m state --state NEW -j DROP
iptables -I INPUT -i br1 -p udp --dport 67 -j ACCEPT
iptables -I INPUT -i br1 -p udp --dport 53 -j ACCEPT
iptables -I INPUT -i br1 -p tcp --dport 53 -j ACCEPT

Test