Sampling frequency on Radio programmes

[Jim web]

In article <553370DA.90207 at soulman1949.com>, Alan Milewczyk
<alan at soulman1949.com> wrote:
> I've noticed that recently the sampling frequency on Radio programmes
> has changed in the last few months from 44.1kHz to 48kHz. I always
> thought 48kHz was only used as the audio component of video files
> whereas 44.1kHz was the standard for standalone audio.

Short answer:

Yes the files available did change a while ago.

Longer answer:

The background is more complex.

For many years the internal audio standard for the BBC has been 48k/24bit.
This is true for sound radio as well as TV. Sent usually over AES3 LPCM
links. That's what has been, and still is, fed into the iplayer from the
radio stations. So far as possible the BBC tended to standardise on that
rate, albeit reducing the nominal depth to 16 bit for domestic users - e.g.
DAB sends 48k/16. (Or 32k/14 for FM as they still use NICAM distribution.
8-] )

When the BBC iPlayer 'Coyopa' system was established a few years ago the
people involved found that the then-current Flash plugin that those
accessing with web browsers depended upon couldn't pass 48k. It downsampled
it to 44.1k. So at that time the people involved arranged for Coyopa to do
a better conversion at their servers than could be expected from the Flash
plugin. IIRC this was done by the 'nemesis' people who were involved, and I
spoke with them about it at the time.

The new 'Audio Factory' is in the process of replacing the now-old
'Coyopa'. And as this was done the new team simply adopted 48k thoughout as
the default. Its clear now that they didn't realise what had been done as a
way to dodge the 'Flash problem' for Coyopa.

I've been told that Adobe told the BBC team that "Flash is now OK for 48k".
What no-one seems to have noticed is that the *Linux* Flash version is
still the 'old' one that converts incoming 48k to 44.1k because it can't
cope with outputting 48k.

However the newer Windows/Mac Flash system are said to be able to pass 48k.
I can't be sure of that for reasons outlined below. And it is *very* easy
for someone to reach the wrong conclusion about it. It seems even Adobe
haven't paid too much attention to the issue. Perils of closed source...

The confusion has been that this issue was forgotten, and then it was
hidden by the tendency of computer OS's of running a mixer or audio server
that simply resamples things to fit its chosen 'baserate'. So thoughout all
the above, various people got 48k regardless *because a mixer system
running on their machine* simply turned 44.1k output from Flash back again
into 48k. This didn't avoid any degrading caused by Flash, just hid the
changes in rate from the user. And, indeed, from Adobe and the BBC in due
time...  :-/

So what you got/get depends on which OS you use, and if it is running a
mixer/server system. (Pulse audio is the main culprit here for Linux. I
can't comment in detail on Windows or Macs as I've not used either for many
years.)

I did the webpage on the Audio Factory a while ago. Since then the BBC
people I know have checked this. And have confirmed that, yes, the Flash
plugin for Linux *still* coverts to 44.1k even if running a pulse audio
mixer/server turns that back into 48k before it hits the soundcard
hardware. To determine this you either have to check what pulse is
*accepting* or do as I do - remove pulse entirely from any Linux system I
use because its a PITA.

When I first started testing and experimenting with using get-iplayer some
months ago it was when the Coyopa system was running. And files obtained
with get-iplayer were 44.1k. But when the main switchover was made, these
changed to 48k and have remained so since. Again, talking to BBC contacts
this seems to be because get-iplayer was previously getting the
'pre-converted 44.1k files' that Coyopa was creating to send out to avoid
the old Flash limit. Now that conversion isn't being done. All the files
are 48k.

So at present we are in a race. We can use get-iplayer to obtain the 48k
and this may be 'better quality' than the old 44.1k. But at some point RTMP
will cease. The hope for browser access is that by then we'll have HTML5
instead which copes happily with 48k, and we can all forget Flash entirely.

I can't be sure what the situation actually is for Windows/Macs. Someone
would need to probe that having ensured that no mixing is being done that
covers the reality.

FWIW I have stayed in contact with some BBC people and hope to do more
tests. I'm also keeping them aware of the situation for Linux users, etc.

The endgame should be that we can now all get better quality 48k. But
negotiating the next few months of changes may be tricky. However at least
I've been able to draw some issues to attention, and that may help smooth
things.

That said I do hope that a new get-iplayer equivalent that can work via a
non-RTMP route will be possible.

Jim

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