320kbps radio audio available via MPEG-DASH

CW christopher at custommade.org.uk
Sun Jan 3 08:00:40 PST 2016

>> What I have done is compared Radio 2 FM vs. DAB vs. 128kbps AAC
>> internet, and I can assure you the DAB is clearly the worst 
>> sounding.
>> The FM vs. 128kbps AAC internet are different, neither is better or
>> worse and it does depend on how good your FM feed is.
>> I can do the FM vs. DAB comparison double blind, because my Arcam
>> FM/DAB tuner has a button to switch between FM and DAB last 
>> stations.
>> If I press it many times very quickly and don't look at the display 
>> I
>> lose track of which it is playing and there's nothing to give it 
>> away
>> if I cover the display with cardboard. Then I listen, make decisions
>> about the sound, and remove the cardboard. I can always tell which 
>> is
>> DAB and which FM, with the exception of Radio 3 when it's DAB is
>> running at 192kbps MP2 and then I can't tell which is FM or DAB. 
>> Which
>> clearly demonstrates the problem with DAB currently is the bit rates
>> are far too low.
> Please could we not have a golden ears debate here.  Just stick to 
> the
> facts of what is available and how it can work or be used with GiP.
> Alastair.

A slight aside, but relevant I feel. Hopefully this puts all the "x 
sounds better than y" debates to rest.

320k or even 128k AAC web streams will always trump DAB or FM for 
national radio irrespective of what golden ears profess to hear. I'm 
perfectly happy with the HLS streams, they're as close to unprocessed 
source audio as we're going to get.

UK DAB's problem is it uses an ancient codec and is tightly constrained 
on available bandwidth. Back when BBC DAB had a massive dollop of 
spectrum and just a handful of channels test TXing, it sounded superb.

Audio Factory doesn't have FM rolloff. The digital playout system used 
by network (and, gradually, Local Radio) which runs at 48 kHz and will 
be almost always playing digital audio either from CD or PCM file, 
unless someone goes and transcodes a dodgy audio file. Excepting poor 
quality source material, with Audio Factory, audio in = audio out (not 
including whatever perceptual frequency magic occurs inside the AAC 
encoders). Prior to reaching AF's encoders, it'll be distributed 
internally as AES or some other lossless method.

The caveat is that processing is still performed prior to them reaching 
AF, but it's not necessarily the same as FM processing. Processing for 
AF is treated with a lighter touch AIUI.

FM has always, and will always, roll off at 16 kHz. It has shaped 
dither (random noise) and signal pre- and de-emphasis to counter its 
varying SNR across the spectrum. It also undergoes FM processing, the 
specifics of which differ across stations. Aside: if you have a good 
system, wait until Radio 1 and 1Xtra are simulcasting the same programme 
and A/B them (online or on DAB if you have a receiver). On a good system 
it's interesting to note how the processing differs.

On an organisational level, national radio is a simpler affair than LR. 
National will process for FM, DAB, online etc but it seems pretty well 

Local Radio due to its history differs qualitatively depending on many 
factors. Whilst all LR stations are now being offered through Audio 
Factory, the means by which the audio reaches AF encoders differs 
greatly depending on age of kit, age of station, what circuits and 
codecs the web stream feeds use etc.

Work is getting underway to analyse and coordinate the processing 
performed to LR station audio - some stations process locally for their 
web streams, some stations 'follow FM', some stations' web streams are 
unprocessed bar simple gain adjustment, it's a bit of a mixed bag.


*Radio 4 is an example of an exceptional case - for a handful of 
important programmes like the Archers, due to legacy constraints like 
their expansive SFX collection, all at 44.1 kHz... But you'd never know 
unless I told you! It also poses other issues for things like picking 
which frequency you run your core audio router at, particularly when you 
have the potential for audio interconnections between production 
facilitiess, each of which are being clocked to (potentially different 
rate) master references...

As far as playout is concerned, Radios 1-7, 1Xtra and Radio London use 
dira, so all 48 kHz. World Service are the notable exception - last I 
saw they're still using Pharos, could be either sample rate.

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