Remove CRC from Audio Payload Frame

Hi guys,

I suggested a while ago removing the CRC from the audio frame in order to reclaim more bits for error correction, changing the puncture table.

I’d like to revisit that. Today, the CRC provides little value. I only use it to check whether the end of stream (EOS) bit is valid. Otherwise it is ignored. The audio data is passed as is to the Codec2 decoder whether the CRC is valid or not.

Using a 16-bit CRC to protect one bit is not a good use of bits. Our decoders have to be able to handle an end of stream condition when the EOS bit is incorrect in any event. The lack of a valid sync word on the next frame has the same effect.

Encryption is the only place where a frame number is meaningful. When using encryption the decoder can track the frame numbers and can detect a valid frame sequence.

If we drop the CRC, we will be encoding 18 bytes, 144 bits + 4 flush bits into 148 * 2 = 296 bits. This needs to be punctured to 272 with a simple 11/12 puncture matrix. The current puncture matrix is ~ 5/6.

Kind Regards,

Rob Riggs WX9O
Mobilinkd LLC

Obviously, there’s a tradeoff between having a CRC and having extra error correction capability. I think it might be a good idea to get rid of the frame CRC.

How did you puncture 296 bits into 272 with an 11/12 matrix?

I tend to agree with all that, although keep in mind that these frames may be transported across the internet in UDP packets where there is not a guarantee of order so frame numbers become important for reordering packets. Maybe this transport will only happen at the super frame level.

There is also a case where you may have multiple receiving sites that are receiving the same transmissions from a single transmitter and sending the duplication of frames to a repeater controller which then combines the streams to produce a more reliable copy should one of receiving site be experiencing fading. (Mobile transmitter situation)

The unique identification of frames and frame order from the transmission source then becomes important.