FFmpeg DeckLink Audio Channel Mapping and Levels

This article provides a straightforward guide on how to configure audio channel mapping and adjust audio levels when outputting media via Blackmagic Design DeckLink cards using FFmpeg. You will learn how to use FFmpeg’s audio filters and mapping parameters to route specific audio channels to your DeckLink output and precisely control their volume levels for professional broadcast setups.

Prerequisites for DeckLink Audio

When outputting to a DeckLink device, FFmpeg requires uncompressed PCM audio. You must define the audio codec as pcm_s16le or pcm_s32le depending on your hardware requirements. Additionally, DeckLink devices generally expect a standard number of audio channels (typically 2, 8, or 16 channels).

To set the output format and device, use the -f decklink muxer along with your device name:

-c:a pcm_s16le -f decklink "DeckLink Duo 2"

Configuring Audio Channel Mapping

To map specific input audio tracks to specific DeckLink output channels, use FFmpeg’s pan audio filter. This filter allows you to define the exact channel layout of the output stream and decide which input channels feed into them.

Example 1: Mapping Stereo Input to 8-Channel DeckLink Output

Many SDI setups require an 8-channel audio configuration, even if you are only outputting stereo. The following command maps a stereo input (Left/Right) to channels 1 and 2 of an 8-channel DeckLink output, leaving channels 3 through 8 silent:

ffmpeg -i input.mp4 -vf format=yuv420p -af "pan=8c|c0=c0|c1=c1" -c:v rawvideo -c:a pcm_s16le -f decklink "DeckLink Studio 4K"

pan=8c: Defines an 8-channel output layout.
c0=c0: Maps input channel 0 (Left) to output channel 0 (SDI Channel 1).
c1=c1: Maps input channel 1 (Right) to output channel 1 (SDI Channel 2).

Example 2: Mapping Multiple Discrete Audio Streams

If your input file has multiple separate audio streams (for example, two discrete mono streams for English and Spanish), you must first merge or map them before outputting.

ffmpeg -i input.mp4 -filter_complex "[0:a:0][0:a:1]join=inputs=2:channel_layout=stereo[out_audio]" -map 0:v -map "[out_audio]" -c:v rawvideo -c:a pcm_s16le -f decklink "DeckLink Mini Monitor"

Adjusting Audio Levels

To modify the volume of your audio channels before sending them to the DeckLink output, use the volume filter. You can specify volume adjustments using decibels (dB) or decimal multipliers.

Example 1: Applying Global Volume Adjustment

To lower the overall volume of your output stream by 6 decibels:

ffmpeg -i input.mp4 -af "volume=-6dB" -c:v rawvideo -c:a pcm_s16le -f decklink "DeckLink Duo 2"

To increase the volume by 1.5 times its original strength:

ffmpeg -i input.mp4 -af "volume=1.5" -c:v rawvideo -c:a pcm_s16le -f decklink "DeckLink Duo 2"

Combining Mapping and Level Adjustments

You can chain the pan and volume filters together within a single audio filter chain (-af). Filters in a chain are separated by commas and executed sequentially.

Complete Integration Command

The following command maps a stereo source to an 8-channel output layout, routes the stereo input to channels 1 and 2, and reduces the output volume by 3dB:

ffmpeg -i input.mp4 \
  -c:v rawvideo \
  -af "pan=8c|c0=c0|c1=c1, volume=-3dB" \
  -c:a pcm_s16le \
  -f decklink "DeckLink Duo 2"

By utilizing these command-line structures, you can ensure your Blackmagic DeckLink hardware receives the exact audio channel configuration and volume levels required for your production environment.