Hi Макс, Nice work on your tests! OK so this is what I hear:
1/ On the noise free English samples, codec 2 at 1200/2400 is slightly worse than MELP at the same rates. This is consistent with other tests (e.g. academic papers using the two codecs as references). 2/ With noisy speech the MELP samples appear to have removed the interfering noise - the noise is suppressed in the output samples, and not faithfully reproduced. This suggests the MELP implementation you have used also has a pre-processing step to remove noise. So it's not quite an A/B comparison - the core MELP codec is being tested on audio that has had the noise suppressed. With noisy input speech, the Codec 2 samples are distorted and unpleasant to listen too, but still (as you suggest) intelligible. For noise suppression in Codec 2 applications we use the Speex noise suppressor (codec2/misc/speexnoisesup.c for a command line version). 3/ It's difficult for me to evaluate the Russian samples, as I don't speak the language, but thanks for the feedback. Yes, Codec 2 was developed on just English samples. 4/ You are correct Codec 2 1200/2400 hasn't been actively developed in over 5 years, recent efforts have been at lower bit rates, and LPCNet (at 1733 bit/s). In particular to support digital voice systems for HF radio. Cheers, David On 11/1/20 11:08 pm, Макс Карпов wrote: > Sorry for sending HTML message, I didn't expect it to mess up the archive. > > Plaintext copy of previous message follows: > > ------ > > Hello! > > Recently I had tested Codec2 on audio samples on both English and Russian > languages, using MELPe as a reference. Moreover, I used three noise modes: no > noise at all, gaussian noise and voice noise (e.g. when you are talking in a > crowded room). As I don't have a source code for MELPe 600, I had tested only > 1200 and 2400 bitrates. Some of MELPe tests were repeated in a environment > where encoder and decoder processes were separated to ensure that no audio > information is leaked through internal buffers and structures. > > To my disappointment, however, Codec2 performed much worse than MELPe, > producing yet still intelligible (in some cases), but heavily distorted > result. I made page with original and encoded samples: > https://m-k.mx/static/codec2/. > > I am surprised why Codec2 performs so bad even on high bitrates where MELPe > produces almost identical audio without any audible artifacts. Codec2 also > degrades significantly on non-English samples (VQ codebooks are fine-tuned > for English?). While degradation at noisy samples is expected (especially in > voice noise), Codec2 sometimes produces very distorted and hardly > intelligible results, while MELPe exhibit only slight degradation. > > So my questions are: > > 1. Are my experiments correct? Maybe I'm missing something important, like > equalizer in front of Codec2 (--eq option appears to have no impact on the > quality). > 2. Are high-bitrate modes like 2400 and 3200 still up-to-date? I have seen a > lot of effort put into 700C mode, but sometimes high-bitrate modes are useful > as well. > > > _______________________________________________ > Freetel-codec2 mailing list > Freetel-codec2@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > _______________________________________________ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
