Re: [vdr] Identifying MPEG-4 HE-AAC (LATM, LAOS) audio formats

[schorpp <thomas.schorpp@gmail.com>]

Am 06.12.24 um 21:32 schrieb Marko Mäkelä:
> A couple of weeks ago, Finland took the penultimate step to get rid of 
> DVB-T. Since years, we had DVB-T and DVB-T2 running in parallel. The 
> DVB-T transponders should be shut down in 2025.

same here but h.265 video and aac audio.

> 
> The step was to introduce a second free-to-air DVB-T2 bouquet to carry 
> some channels that were previously only available with DVB-T and MPEG-2. 
> The VDR channel search failed to find this transponder; I ran a patched 
> version of https://github.com/mighty-p/t2scan/pull/15/ to generate a new 
> channels.conf. (Is this a bug of VDR, or a feature? I remember that when 
> I originally set up VDR, I had to get a channels.conf from tscan or 
> t2scan.)

have You tried the latest git version of scan in dvb-utils @linuxtx.org 
repo?

> 
> Unlike our first free-to-air DVB-T2 bouquet, which uses MPEG-4 H.264 
> video with AC3 audio, the second one uses H.264 video and HE-AAC audio, 
> also known as LATM or LOAS. This had to be explicitly enabled for my 
> output device: https://github.com/reufer/rpihddevice/issues/20
> 

> The HE-AAC support in rpihddevice is experimental, 

as it has been in ffmpeg until today and rpihddevive is based on it?

> maybe because the 
> sampling rate is hard-coded as 48 kHz and the number of channels as 2. 
> Unlike AC3, it does not look like these parameters would be available in 
> any audio frame header. As far I understood from a NorDig.org 
> specification, the sampling rate would always be 48 kHz, but the number 
> of channels could be identified by something in the container, 
> specifically, Table 12.28 and Table 12.29 at page 164 of
> https://nordig.org/wp-content/uploads/2016/03/NorDig-Unified- 
> Requirements-ver.-3.2.1.pdf

Have You tried the latest git ffprobe version, does it detect the aac 
flavor of Your dvb-t2 DVB channels correctly?

Have You tried latest git mediainfo?

> 
> What would have to be changed in VDR or rpihddevice or both to correctly 
> switch between 2-channel and 5.1-channel HE-AAC audio? This is mostly an 
> academic question and exercise, because I only have stereo audio 
> hardware and don't know if any programming is actually available with 
> multichannel audio.

Not at all academic. Since AVRs do not implement Fraunhofer codecs we 
need a HDNI PCM Multichannel driver for the raspberry pi to playback 5.1 
channel aac without transcoding it to ac3 which can be done with mplayer 
on the fly.

I've made a HDMI PCM Multichannel output to AVR driver patch for the 
Allwinner SoC alsa audio driver, look for it on the alsa-dev list archives.

> 
> Best regards,
> 
>      Marko

y
tom