Tech Tip: Using Multichannel Audio in Distributed Systems

​Surround sound has been used in home theaters for decades, imitating realistic environments and helping consumers feel even more immersed in the content that they view. While this is great for a dedicated home theater, problems arise when distributing this multichannel audio to multiple zones with varying capabilities. Whether it is no sound at all or missing voices, this is mainly seen when a room's audio varies between using AVRs and Preprocessors or where audio is played back by the display.

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The varying types of Multichannel Audio

To understand these issues, we need to understand the two main types of Multichannel Audio:

• Pulse Code Modulation (PCM): Uncompressed digital representation of analog audio sent in individual channels (No Encoding)
• Bitstream: Compressed encoded audio that must be decoded for a device to play correctly, to the device's configuration of channels. Such as Dolby or DTS.

As said above, the main difference between multichannel audio formats is that some are encoded, and some are not. But how does this affect your system?

Understanding the Issues of Multichannel Audio and how to resolve them

The following issues and resolutions are situations in which a zone with a multichannel audio player/decoder is functioning correctly. If you are experiencing issues with an AVR or Preprocessor, check the device's configuration settings and ensure that the channel configuration is set accordingly.

Issues with PCM audio:

• Display plays audio but is missing voices: Because PCM does not have to be decoded, displays will simply play the channels that they have. In Multichannel PCM, the center channel will mainly carry the voices, and because most displays are meant for stereo (Left and Right) audio, the display will simply not play the center channel. AVR's and Preamplifiers may also face this issue if improperly configured. This can be resolved in two ways:
  
  1. Using Bitstream Audio. Most modern displays can decode bitstream audio formats and properly decode the audio to the display's correct channel configuration.
     
  2. Using a Downmixing Device. Most downmixing devices can take Multichannel Audio whether it is bitstream or not and change the output format to 2 Channel Stereo PCM.
     
• Distributed Audio plays audio but is missing voices: Like above, most distributed audio systems can only play two-channel audio and will only accept the Left and Right channels of a multichannel PCM signal. We can resolve this in two ways:
  
  1. Setting sources to stereo PCM audio. While this may compromise the Multichannel zone(s), it will allow you to distribute your source's audio without any additional costs.
     
  2. Using a Downmixing Device. Most downmixing devices can take Multichannel Audio, whether it is bitstream or not, and change the output format to 2 Channel stereo PCM. Most downmixing devices can extract stereo PCM audio, allowing you to go directly into the distributed system.

Issues with Bitstream Audio

• The display does not play audio or plays incorrectly: While most displays can decode bitstream audio, they may not be able to decode High Bit Rate formats. You may get an on-screen notification or no audio at all. There are three possible resolutions:
  
  1. Decreasing the Bitstream Format. Setting the source's audio to a lower bitrate format may place the audio within the display's decoding capabilities while allowing the other multichannel zone(s) to maintain the surround.
     
  2. Changing the format to Stereo PCM. If a display cannot decode bitstream audio, then you will have to provide it with a Stereo PCM signal to maintain audio in that zone.
     
  3. Using a Downmixing Device. Once again, the downmixing device is the true solution. Almost all audio playback devices can play a stereo PCM signal.
• Distributed Audio does not play: Distributed audio systems do not accept bitstream audio for the most part. You may experience clipping sounds or no audio at all. There are only two resolutions:
  
  1. Changing format to Stereo PCM. Referring to distributed audio's preference for Left and Right only audio, decreasing to the most basic signal should resolve your issue.
     
  2. Using a Downmixing Device. To maintain any multichannel audio with most distributed systems, you will need a device that can change the signal to an acceptable format. Some HDMI Distribution devices may have downmixing already built-in, allowing seamless integration.

​We are always trying to provide the best experience for a client by understanding the issues that can arise with multichannel audio. We can address problems when designing the original solution, saving both you and your clients time and headaches, as well as reducing the number of truck rolls.