Weird volume behaviour

[Rodders53]

Dolby Digital 2.0/5.1 transcoded from the HE-AAC stream in the HD mux may well be mixed to a different standard: (Film mix vs TV mix is 8dB lower, I believe)...

Thus a PCM stereo SD channel may well sound louder than a DD channel on HD when the HDR-Fox-T2 is set to output surround.... That is what the volume control (of the SD channels in PCM is for... to match the DD HD channel levels).

For many of us max could be close enough...
IIRC once upon a time I had it under max for channel surfing when using the PVRs in surround (though it could well have been the Foxsat-HDR rather than the -T2)... but found that the decoding delays were "annoying" when switching between DD and PCM. So we use the stereo decoding setting for HD most of the time.

NB for the BBC (as I understand it) the exact same transport stream includes Dolby-E encoded sound data that is sent to both DVB-T and DVB-S coding and multiplexing sets... Satellite uses Dolby proper and Terrestrial is transcoded (with an ability to mix in Audio Description - that is carried on satellite as a separate stereo PCM stream)...
AIUI the down-mix from 5.1 to stereo is based on the metadata in the Dolby E stream... and set by the programme producers... It certainly will NOT remove the surround (or LFE) channels (they may contain important information for the production!!).
Quite often the mixes are such that good ole Dolby Surround receivers can recover phase-encoded differences very effectively to steer sound in a very effective pseudo surround way.
Useful on iPlayer, for instance.

NB The same tech standards for video and audio delivery to playout is used by all the PSBs...

On satellite all C5HD content is flagged as 5.1... whether a stereo mix or not. Go figure.

 

[Black Hole]

Firmware revision makes a difference – in the early days of the HDR-FOX, the volume difference between StDef and HiDef was much worse.

 

[Owen Smith]

Dolby Digital 2.0/5.1 transcoded from the HE-AAC stream in the HD mux may well be mixed to a different standard: (Film mix vs TV mix is 8dB lower, I believe)...

The normal audio on Freeview HD is AAC-LC (Low Complexity) which is the correct thing to use given the bit rates. Only audio description uses HE-AAC (High Efficiency) which is mixed in to the normal audio if enabled. The HDR Fox T2 mucks up the relative volumes of that mixing I believe unless you use very old Humax firmware (1.02.20 if memory serves correctly).

Thus a PCM stereo SD channel may well sound louder than a DD channel on HD when the HDR-Fox-T2 is set to output surround.... That is what the volume control (of the SD channels in PCM is for... to match the DD HD channel levels).

I've never thought of the Humax volume control as being there to match the SD channels volume to the HD channels. Why would I want to degrade the SD audio quality by running it through DSP operations to reduce the LPCM bit depth? (which is what will happen in effect, 8db down will lose nearly 3 bits).

NB for the BBC (as I understand it) the exact same transport stream includes Dolby-E encoded sound data that is sent to both DVB-T and DVB-S coding and multiplexing sets... Satellite uses Dolby proper and Terrestrial is transcoded (with an ability to mix in Audio Description - that is carried on satellite as a separate stereo PCM stream)...

Satellite still has to transcode, it just does so from Dolby E (which is a high bit rate format intended for production environments) to Dolby Digital while Freeview HD transcodes from Dolby E to AAC. Dolby Digital is no more or less "proper" than AAC, and Freeview uses AAC because it is a more efficient codec than Dolby Digital (satellite can afford higher bit rate for the audio).

AIUI the down-mix from 5.1 to stereo is based on the metadata in the Dolby E stream... and set by the programme producers... It certainly will NOT remove the surround (or LFE) channels (they may contain important information for the production!!).

Indeed, or at least it shouldn't lose anything if working properly. The Dolby E downmix metadata to stereo will tell the Freeview HD transcode how to generate the AAC stereo stream, and then the transcode has to work how how to add the extra AAC data to convert that to 5.1. Dolby Digital works the same way, there's a stereo stream embedded in it and data that converts that to 5.1. This is the only way to get both stereo and 5.1 reliably to consumer grade equipment, at least how to without having entire duplicate audio streams for stereo and 5.1 which would be prohibitive on bit rate.

On satellite all C5HD content is flagged as 5.1... whether a stereo mix or not. Go figure.

Now that's a pain in the arse since many AVRs cannot be told to do a Dolby Surround decode of a 5.1 signal. To work around that will require either forcing the satellite receiver to output stereo over optical/HDMI or using analogue stereo outputs.

 

[Owen Smith]

Firmware revision makes a difference – in the early days of the HDR-FOX, the volume difference between StDef and HiDef was much worse.

I don't think I was using one that early, I don't remember any such change and I've had a home cinema amp with DD 5.1 input over optical since 2006.