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

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

[schorpp]

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

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

[Marko Mäkelä]

Fri, Dec 06, 2024 at 10:31:06PM +0100, schorpp wrote:
>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.

Right, it's H.265 video in Germany. I found somewhere a reference to 
something that would transcode H.265 into H.264 on the fly on a 
Raspberry Pi 3, so that the Videocore GPU could play it in real time.

>>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?

At https://git.linuxtv.org/v4l-utils.git I found a reference to the 
command dvbv5-scan. An "apt search dvbv5-scan" on my Debian Sid as well 
as on Raspbian 12.8 yielded two packages: dvb-tools and w-scan-cpp. I 
vaguely remember that I had used the latter one a few years ago. I just 
invoked the following on the Raspberry Pi:

w_scan_cpp -c FI

The output during scanning looks convincing, but the program seems to 
get stuck waiting for something at the end, for several minutes.

I also tried to invoke dvbv5-scan from dvb-tools, but it seems to 
require a file that contains some initial parameters.

Anyway, I was mainly wondering if it is normal that the EPG update in 
VDR did not find the new transponder. I guess it is; the search space 
would be huge if it had to cover satellite, cable, terrestrial and some 
IPTV sources as well.

>>The HE-AAC support in rpihddevice is experimental,
>
>as it has been in ffmpeg until today and rpihddevive is based on it?

As in:

#ifdef ENABLE_AAC_LATM
#warning "experimental AAC-LATM frame parser enabled, only 2ch/48kHz supported!"
#endif

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

I only have 2-channel recordings or DVB-T2 streams available to me.  
Those already play without any problems. But I don't have any recording 
with multichannel audio. I would like to implement correct detection and 
playback of multichannel HE-AAC audio, so that the AAC audio support 
could be enabled by default in rpihddevice.

>Have You tried latest git mediainfo?

I was reading the libmediainfo source code a few days ago. I guess I 
should just have executed mediainfo on an MPEG-4 TS file inside a 
debugger, to find out how and where exactly it detects the audio 
parameters.

>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.

Right, also according to the NorDig specification, some transcoding of 
audio is mandatory for some audio output options.

>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.

Sorry, I could not find it. I figured out that AVR here must refer to 
Audio Video Receiver. The only HDMI output devices that I have are 
displays with 2 built-in speakers. I believe that I am currently using 
2-channel PCM audio output.

Are there any sample MPEG TS recordings with 5.1 channel AAC audio?  
That would be very helpful. In a 2-channel setup, would it be customary 
to pass through only the first 2 channels and ignore the rest?

	Marko