In article , Don Pearce
wrote:
On Fri, 6 Aug 2010 20:48:21 +0100, tony sayer wrote:

In article , Peter Chant
scribeth thus
Jim Lesurf wrote:


Does it? Stereo FM is limited to about 15kHz. I thought DAB exceeded
that. It does use a 48k sampling rate so in principle could reach
24kHz, although I assume the data-reduction will throw away some of
the HF end depending on settings and content.

Where did you get the info? I've not measured DAB in ages. I guess I
should check.

I've not read it for a while but I through FM was 17.5 - 18 kHz

Its cut off at 15 kHz in most all systems to protect the pilot tone at
19 kHz..


The low cutoff in FM is not so much to do with the pilot tone as the
fact that it is distributed to the transmitters using NICAM, which has a
32kHz sampling rate.

I think this is a situation that have 'sort of evolved' over the years for
a variety of reasons. (Bit like AM on MW where bandwidths decades ago were
higher than nowdays.)

In days of yore with mono it seems likely that BBC FM could at times give
an audio bandwidth above 15kHz as there was no pilot tone, etc. However as
has been mentioned that was all at the mercy of the GPO landlines. So while
people getting Wrotham may have had good results, people up here in the
frozen North were likely to get something rather more limited, with added
noises, phase oddities, etc, etc.

Stereo complicated the issue, and IIRC that was when people decided to cut
the audio at 15kHz (ish) to give space for the 19kHz tone. Probably more
important then than now as the early decoders had to use simple tuned
circuits to isolate the 19kHz and double it to get the 38k subcarrier. So
any nearly audio could upset that quite easily. (Took a few years before
good PLL decoders were common and decoders became fairly resistance to
near-19k distractions.)

Then the BBC started using LPCM to get over the pest of GPO analogue
landlines. IIRC they used one system for a time before changing to NICAM,
and have probably changed details. But as has been said that pretty much
nailed a hard limit of 16kHz onto the system.

I don't know the current practical bandwidth for DAB broadcasts. Must
measure it unless someone knows of some recent results (?) In principle it
might be as high as 24 kHz. But the data reduction probably tends to
discard stuff above 15kHz (ish) as being 'inaudible' and prefers to give
the bits to lower frequency components with a higher priority.

FWIW Pure used to boast of a fancy 'technology' they had to 'improve' the
sound of DAB. Gave it a nice PR name I've forgotten. But when I asked them
to explain they never did... Odd that. :-)

Slainte,

Jim

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