[H-GEN] Re: converting from speex to ogg vorbis

Gavin Unsworth gavin at helot.net
Tue Sep 14 22:04:05 EDT 2004


> > Digital audio is a signal level sampled at a given frequency.  When
> > people talk about 44KHz audio, that refers to the frequency: 44,000
> > samples per second.  CD audio is 44.1KHz, telephones operate somewhere
> > around 8KHz, FM radio is 22KHz or so IIRC.
> 
> Not sure why you'd include FM radio here, as it is not digital (at
> least in this country).  I'm guessing that you were referring to the
> digital telephone system.  It's not really useful to compare analogue
> frequencies with digital sampling frequencies, though there are a few
> similarities.

Being a pedant for anyone who cares ..... analogue telephone channels have a
BW of about 3kHz (300Hz - 3.3kHz). Back in the day, the Nyquist theorem was
taken literally and digital systems sampled at the Nyquist rate (twice the
maximum frequency in the source signal being the minimum sampling rate).

For ISDN, the maximum analog freq of 4KHz was assumed - sampled at the
nyquist rate (8kHz), 8 bits per sample = 64kbps. This is why an ISDN B
channel is 64kbps - to carry a single uncompressed voice channel.

FM Radio has a BW of about 15kHz. 50Hz - 15kHz. Digital Radio is on-air in
Sydney as a trial now. This is the second trail in AU, the first being in
Melbourne in 1995. Unless I'm mistaken about the current trial, both
implemented the Eureka 147 system. This system allows for channels with
differing quality within it's 1.2Mbps (approx) "payload".

<trivia>
Whilst technically brilliant and beyond it's time (research on Eureka 147
created MUSICAM encoding which became MPEG-2 Layer II .... and without that
we would have never had MPEG-2 layer III (mp3!)) Eureka 147 usually uses
frequency spectrum in L-band (around 1.4MHz).

When the first trial was conducted, L band was earmarked for DAB (Digital
Audio broadcasting) using Eureka 147 in Australia. Political pressure from
well connected media owners in AU led the ABA to abandon DAB planning and
instead make a quantum leap to Digital TV planning. This has had numerous
flow-on effects ... one of which was that L-band has since been used for
other purposes.

It's likely that we will end up with DAB, but using a different system known
as IBOC (in band on-channel) .... US technology and not as elegant as the
European Eaureka 147.
</trivia>


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.759 / Virus Database: 508 - Release Date: 9/09/2004
 





More information about the General mailing list