[opus] OPUS on cortex M4 (Nicolas Ehrenberg)

Ehrenberg Nicolas enicolas at student.ethz.ch
Sun Jul 1 15:57:34 UTC 2018

Thanks for the reply. 
For my application I unfortunately need a better signal reconstruction. It's not necessarily a problem that the constant DC voltage is removed, but the audio signal will need to be more exact because it's also studied visually. To be more exact, I need to record and transmit audio data recorded from animals (mostly birds).

Are there ways to achieve a more similar output signal compared to the input signal? 

As I already mentioned I tried different complexities which did not lead to much improvement. If I input other data values,  e.g, 2000 +- 500, or 2000 with occasional dips and peaks, I still do not get a reasonably good visual signal reconstruction.   

Also, I'm not quite sure if my opus implementation works as it should yet. I can run the encoder with OPUS_APPLICATION_RESTRICTED_LOWDELAY, but If I try to run it with OPUS_APPLICATION_VOIP or OPUS_APPLICATION_AUDIO the opus_encode function returns just 2 (size of encoded samples) and the opus decode just returns 0's. I would imagine to get the best audio reconstruction with OPUS_APPLICATION_AUDIO.
What could be the reason for OPUS_APPLICATION_AUDIO not working?


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   1. Re: OPUS on cortex M4 (Timothy B. Terriberry)


Message: 1
Date: Fri, 29 Jun 2018 14:41:07 -0700
From: "Timothy B. Terriberry" <tterribe at xiph.org>
To: Ehrenberg Nicolas <enicolas at student.ethz.ch>, "opus at xiph.org"
        <opus at xiph.org>
Subject: Re: [opus] OPUS on cortex M4
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Ehrenberg  Nicolas wrote:
>      for(uint16_t i = 0; i < test_data_size; i++)
>      {
>      test_data[i] = 100;
>      }
> [snip]
> The decoded values start out at 0, go to -16 up, then up to 135 and then
> slowly down to the 17 (see attached file).
> Furthermore, I do not see much of a change if I increase the complexity.

You're encoding a signal with a constant (DC) offset. opus_encode()
includes a 3 Hz highpass filter to remove the (inaudible) DC component,
because it screws up other parts of the codec (mainly it would create
MDCT leakage that is completely unmasked). So you shouldn't expect to
get your constant values of 100 back.

In general the numeric values of the output aren't guaranteed to
resemble the numeric values of the input. The codec will just try to
make the two signals sound the same to human ears.


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