Fine Business Adrian, yes I've found the Codec 2 modes are usable with a
few percent bit errors. I actually have a function called "ear
protection" in Codec 2 to avoid some of the wilder excursions due to bit
errors.
The use of compression is interesting. I've been thinking of using
compression and equalisation to limit the dynamic range of the speech
signals entering the codec, which will make them easier to code,
especially at a low bit rate.
Cheers,
David
On 17/4/21 11:02 pm, Adrian Musceac wrote:
Hi,
I have done some investigation recently into the comparative bit error
handling of Codec2 and Opus over the air with a VHF modem. I have
saved some audio files from the results which can be loaded in Audacity.
Codec2 samples had a bitrate of 1400 bps, Opus samples had a bitrate
of 9400 bps. Both codecs were used with a 40 ms frame duration, Opus
was configured with fixed bitrate and no DTX.
For both codecs I have recorded samples with and without audio
compressor on.
My conclusion is that Codec2 handles bit errors in the audio stream a
lot better than Opus with default settings as you can see in the wav
files below.
Opus has high pitch high amplitude artefacts which are difficult to
listen to when bit errors are present in a default configuration.
Using an aggressive audio compressor setting with Opus improves the
response to bit errors quite a lot, while for Codec2 there is not much
improvement added by audio compression, except for a little improved
readability resulting from less dynamic range.
Hope this is useful to someone.
Audio samples:
- no audio compression
http://qradiolink.org/images/opus_errors_no_compressors.wav
http://qradiolink.org/images/codec2_errors_no_compressor.wav
- with audio compression
http://qradiolink.org/images/opus_errors_with_compressors.wav
http://qradiolink.org/images/codec2_errors_with_compressor.wav
Adrian
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