OPUS decoder

RE: OPUS decoder

Michal — Fri, 20 Apr 2018 11:15:38 GMT

Hi - sorry, I somehow missed your question. Yes, that all WAS right. Now I understand OPUS better and I can decode B on nrf and need to add header data to be able to decode A on PC.

RE: OPUS decoder

Filip Kubicz — Tue, 17 Apr 2018 12:56:58 GMT

Please see updated answer.

RE: OPUS decoder

Filip Kubicz — Tue, 17 Apr 2018 12:16:57 GMT

Hi,

1. As Stian said - Opus has 2 algorithms, CELT and SILK. Both of them simultaneously will not fit into the nRF52832 memory, so in Smart Remote you can choose at compile time which one you want to use. The hybrid mode is available to use if only you have enough flash and RAM memory.

2. The Opus decoder does not need additional information to know which algorithm was used to encode the current frame. In the original Opus code (https://opus-codec.org/) there is a function opus_packet_get_mode() which checks if the frame was encoded with CELT or SILK.

You can decode the data encoded on PC - if you open the .opus file and send it frame by frame to nRF52 it is possible to use Opus decoder as provided in Smart Remote code (there is not support for this scenario yet).

Host decoding:
There is the NVS (Nordic Voice Solution) package which is responsible for decoding sound on the host side (Linux). It can be found in downloads section:
https://www.nordicsemi.com/eng/Products/Bluetooth-low-energy/nRFready-Smart-Remote-3-for-nRF52-Series
When you download "Host software", you can see in the source code how the Opus decoder is used to decode sound received from nRF52 / Smart Remote.

Embedded decoding on nRF52:
There is work ongoing to provide downstream audio support.

EDIT: There was some confusion on which header we are talking about:
- Opus file header with encode parameters
- the optional header added to each frame by Smart Remote to be able to easily reassembly sound on host side (in NVS) when streaming. Irrelevant to this question.

RE: OPUS decoder

Stian Røed Hafskjold — Tue, 17 Apr 2018 11:10:52 GMT

The smart remote team is going to look into the possibility of adding downstream audio on the SR3 (decoding). So we can provide you with some more details within a couple of days.

RE: OPUS decoder

Stian Røed Hafskjold — Tue, 17 Apr 2018 11:02:49 GMT

Just to make things clear:

You can encode A on nRF
You can decode A on nRF
You are not able to decode A on a PC
You can encode B on a PC
You are not able to decode B on nRF

Is this correct?

RE: OPUS decoder

Stian Røed Hafskjold — Tue, 17 Apr 2018 10:24:19 GMT

Hello,

Hybrid mode is not supported because then you have to compile in both SILK and CELT which would make the code too big. Also, SILK is using quite a lot of CPU, so we decided to support only either SILK or CELT. So you would need to find a PC tool which supports selecting only one of them.
SR3 is sending only raw compressed data, you don't need any header to decode the data. http://opus-codec.org/docs/opus_api-1.2/group__opus__decoder.html#ga7d1111f64c36027ddcb81799df9b3fc9