TV Audio Bitrates

[RS]

Sent: Saturday, August 26, 2017 00:04  I wrote

>On 25/08/2017 16:28, Vangelis forthnet wrote:
>> ...
>>But still the "Not so" part of his post stands true for various pids 
>>outside the main BBC TV channels...
>> Just try (with 3.02)
> >
>> get_iplayer --type=tv --pid=p059b38x --tvmode=hvfsd
>> (pid expires in 2 days) and you'll discover it fetches as a 128kbps hvf* 
>> mode... One other oddity for this pid is it's not available as legacy 
>> hls* (/flash*) tvmodes, only dvf*/hvf*.
> >
>> I guess a better choice of wording would be:
>> "GiP 3.02 tries to download every hvf* tvmode with a 320kbps audio 
>> stream, whenever that one is available on the CDN server"
>> ...

>As you say, this example is outside the main BBC channels, and more 
>specifically not a regular broadcast. I am sure you have done more analysis 
>than I, but to a large extent regular broadcasts have PIDs beginning with b 
>and special programmes such as Red Button or clips have PIDs beginning with 
>p. In the case of Red Button programmes there is usually no HLS mode.  I 
>think this is the first time I have seen a 125kbit/s audio rate for a TV 
>programme but that may be because when there has been no HLS mode I have 
>usually used HVFxsd rather than a 50fps mode.  That up to now has given 
>96kbit/s HE-AAC v1 in contrast to 96kbit/s AAC-LC for HLShd.  The video 
>also has some unfamiliar settings.
> ...

After posting that I saw that the programme you were referring to was indeed 
a Red Button programme.  I have now downloaded the Red Button coverage of 
the cross-country phase of the Burghley CCI**** horse trials on Saturday, 
b09544c6 in v3.02.  There were no HLS modes other than HLSvhigh.  The audio 
bit rate for HVFxsd was 125kbit/s.

For these programmes I used to use hvfxsd5 which was usually an Akamai HTTP 
CDN.  The number of HVFxsd CDNs seems to have been reduced.  It used to be 
about 8 and was 3 for this programme.  As the programme was not in my cache 
I could not use --info but I was able to download it.  hvfxsd1 and hvfxsd3 
both gave about 17Mbit/s and hvfxsd2 (the last I tried) about twice that.

To sum up
1.  I was wrong about the distinction between b and p prefixes for PIDs.
2.  The rule for audio bit rate seems to be that 320kbit/s audio will be 
used for all HVF modes including HVFlow for programmes which have been 
broadcast on main stream television channels and for Red Button programmes 
it will be 125kbit/s.

125kbit/s audio for Red Button programmes is an improvement over the 
96kbit/s HEAAC v1 which used to be used for some HVF modes and avoids the 
problem of loss of high frequencies with players which do not support SBR.

Best wishes
Richard



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