bit rate challenge
 

Can any of you produce a clean sounding mp3 of a swept frequency test cd ?
the source has to be a commercial cd with a sweep freq track on it. I
suspect not, I played a test cd in a portable cd player with it's anti jog
feature enabled, it sounded dreadfull but music cds still sounded ok. nb the
anti jog spins the cd twice it's normal speed and reads ahead into a buffer
to give it time to recover if it gets jogged, it really messed up the sweep
from 10kHz onwards.

--
Pete Cross

bit rate challenge
 

Pete Cross wrote:
Can any of you produce a clean sounding mp3 of a swept frequency test cd ?
the source has to be a commercial cd with a sweep freq track on it. I
suspect not, I played a test cd in a portable cd player with it's anti jog
feature enabled, it sounded dreadfull but music cds still sounded ok. nb the
anti jog spins the cd twice it's normal speed and reads ahead into a buffer
to give it time to recover if it gets jogged, it really messed up the sweep
from 10kHz onwards.

Surprising. I would have expected the buffer to store the uncompressed
PCM, and potentially improve the sound due to cleaner clocking.

--
Mark.
http://tranchant.plus.com/

bit rate challenge
 

"Pete Cross" 1@2 wrote in message ...
Can any of you produce a clean sounding mp3 of a swept frequency test cd ?
the source has to be a commercial cd with a sweep freq track on it. I
suspect not, I played a test cd in a portable cd player with it's anti jog
feature enabled, it sounded dreadfull but music cds still sounded ok. nb
the
anti jog spins the cd twice it's normal speed and reads ahead into a
buffer
to give it time to recover if it gets jogged, it really messed up the
sweep
from 10kHz onwards.

--
Pete Cross

Pete, I can do a 20-20kHz MP3 sweep, what but rate did you want it done at?
Let me have an email address to send it to.

Note that MP3 (and MP2) use psychoacoustic masking to reduce the bit rate.
This means that the audio band is broken down into many (don't know exactly
how many) individual frequency bands, with the idea that quiet sounds in
bands adjacent to loud sounds will be masked and therefore not codec. The
many filters means that a sweep will be constantly crossing filter
boundaries dynamically, and will pretty much confuse the MPEG coder.

When there was the 199? Football World Cup in France, I sold France Telecom
a large number of automatic audio test meters so FT could test the 400+ ISDN
codecs they bought to provide audio feeds to the world's broadcasters. These
ISDN codecs were being used on MP2 at 128kbps (pretty much as DAB is in the
UK). Using spot frequencies, all tests were fine, as was audio quality
(mostly speech). but as soon as the automated sweep went through, the
frequency response had so many anomalies as to be meaningless. FT solemly
carried out the tests, failed every codec, but used them anyway....aren't
the French wonderful?........

S.

bit rate challenge
 

In article , Serge Auckland
wrote:


Note that MP3 (and MP2) use psychoacoustic masking to reduce the bit
rate. This means that the audio band is broken down into many (don't
know exactly how many) individual frequency bands, with the idea that
quiet sounds in bands adjacent to loud sounds will be masked and
therefore not codec. The many filters means that a sweep will be
constantly crossing filter boundaries dynamically, and will pretty much
confuse the MPEG coder.

Why would that confuse the coder if the level of compression wasn't
excessive? I assume the sweep rate is slow enough that it will take many
'chunks' to cover the entire range.

Don't know the correct values off-hand, but IIRC MPEG type systems tend to
use 512/1024 sample chunks and hence chunk lengths of the order of a few
tens of milliseconds. This in turn implies bandwidths per spectral
component of the order of a few tens of Hz.

When there was the 199? Football World Cup in France, I sold France
Telecom a large number of automatic audio test meters so FT could test
the 400+ ISDN codecs they bought to provide audio feeds to the world's
broadcasters. These ISDN codecs were being used on MP2 at 128kbps
(pretty much as DAB is in the UK). Using spot frequencies, all tests
were fine, as was audio quality (mostly speech). but as soon as the
automated sweep went through, the frequency response had so many
anomalies as to be meaningless. FT solemly carried out the tests,
failed every codec, but used them anyway....aren't the French
wonderful?........

How were the time periods of the FT measurements related to those of the
sampled chunks? I am sure you are correct in that the response will be
affected[1], but I am curious as to how large the effects might have been,
provided the level of data reduction wasn't too servere....

[1] Partincularly as reduction would be aimed at being nominally innocuous
on a *psychoacoustic* basis, not for a spectral analysis. :-)

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

bit rate challenge
 

In message , Pete Cross writes
Can any of you produce a clean sounding mp3 of a swept frequency test cd ?
the source has to be a commercial cd with a sweep freq track on it. I
suspect not, I played a test cd in a portable cd player with it's anti jog
feature enabled, it sounded dreadfull but music cds still sounded ok. nb the
anti jog spins the cd twice it's normal speed and reads ahead into a buffer
to give it time to recover if it gets jogged, it really messed up the sweep
from 10kHz onwards.


I've tried to do this, using the LAME codec at 192k bps. I also
attempted MP3s of white noise and pink noise for speaker analysis. I'd
advise you to forget it. The frequency response you get on playback is
not good enough. (As a result of the perceptual coding algorithm.)
--
Chris Morriss

bit rate challenge
 

"Mark Tranchant" wrote in message
...
Pete Cross wrote:
Can any of you produce a clean sounding mp3 of a swept frequency test cd
?
the source has to be a commercial cd with a sweep freq track on it. I
suspect not, I played a test cd in a portable cd player with it's anti
jog
feature enabled, it sounded dreadfull but music cds still sounded ok. nb
the
anti jog spins the cd twice it's normal speed and reads ahead into a
buffer
to give it time to recover if it gets jogged, it really messed up the
sweep
from 10kHz onwards.

Surprising. I would have expected the buffer to store the uncompressed
PCM, and potentially improve the sound due to cleaner clocking.

That's exactly what I was thinking. Surely the processor power to compress
and decompress it is more expensive than the memory to store it
uncompressed. But the instructions for my Sony CD player claim disabling the
anti-shock feature gives you better sound quality... odd.

bit rate challenge
 

"Jim Lesurf" wrote in message
...
In article , Serge Auckland
wrote:


Note that MP3 (and MP2) use psychoacoustic masking to reduce the bit
rate. This means that the audio band is broken down into many (don't
know exactly how many) individual frequency bands, with the idea that
quiet sounds in bands adjacent to loud sounds will be masked and
therefore not codec. The many filters means that a sweep will be
constantly crossing filter boundaries dynamically, and will pretty much
confuse the MPEG coder.

Why would that confuse the coder if the level of compression wasn't
excessive? I assume the sweep rate is slow enough that it will take many
'chunks' to cover the entire range.

There *was* pretty severe compression, the output from the codec was MP2 at
128kbps, so around 9-10:1 Sweep rates were 1.5, 5 or 20 seconds for a
20-20k sweep. (You may know the Lindos LA100 test set)

Don't know the correct values off-hand, but IIRC MPEG type systems tend to
use 512/1024 sample chunks and hence chunk lengths of the order of a few
tens of milliseconds. This in turn implies bandwidths per spectral
component of the order of a few tens of Hz.

That's what I remember, my MPEG theory is getting a bit rusty. Something
else to read up about!

When there was the 199? Football World Cup in France, I sold France
Telecom a large number of automatic audio test meters so FT could test
the 400+ ISDN codecs they bought to provide audio feeds to the world's
broadcasters. These ISDN codecs were being used on MP2 at 128kbps
(pretty much as DAB is in the UK). Using spot frequencies, all tests
were fine, as was audio quality (mostly speech). but as soon as the
automated sweep went through, the frequency response had so many
anomalies as to be meaningless. FT solemly carried out the tests,
failed every codec, but used them anyway....aren't the French
wonderful?........

How were the time periods of the FT measurements related to those of the
sampled chunks? I am sure you are correct in that the response will be
affected[1], but I am curious as to how large the effects might have been,
provided the level of data reduction wasn't too servere....

The frequency response dips were 10dB+. There were several dips in the
response throughout the sweep. If I did a frequency response at spot
frequencies, if I recall, 1 sec at each frequency, the response measured
flat, but if the frequencies were gliding tones, I got these anomalies.
Codecs were AETA.


[1] Partincularly as reduction would be aimed at being nominally innocuous
on a *psychoacoustic* basis, not for a spectral analysis. :-)

Slainte,

Jim

S.

bit rate challenge
 

Thus spake Serge Auckland:
Snipped
There *was* pretty severe compression, the output from the codec was
MP2 at 128kbps, so around 9-10:1 Sweep rates were 1.5, 5 or 20
seconds for a 20-20k sweep. (You may know the Lindos LA100 test set)

Good heavens, haven't seen reference to the Lindos LA100 since I calibrated
one (pair?) back in 1991 - made one hell of a change from calibrating &
repairing HP8903A/B's or bucket loads of RF generators.

bit rate challenge
 

On Fri, 20 Jan 2006 16:56:11 +0000, Serge Auckland wrote:
snip

When there was the 199? Football World Cup in France, I sold France Telecom
a large number of automatic audio test meters so FT could test the 400+ ISDN
codecs they bought to provide audio feeds to the world's broadcasters. These
ISDN codecs were being used on MP2 at 128kbps (pretty much as DAB is in the
UK). Using spot frequencies, all tests were fine, as was audio quality
(mostly speech). but as soon as the automated sweep went through, the
frequency response had so many anomalies as to be meaningless. FT solemly
carried out the tests, failed every codec, but used them anyway....aren't
the French wonderful?........

Maybe they didn't plan on broadcasting frequency sweeps anyway.
:-)

/Martin

bit rate challenge
 

"Martin Schöön" wrote in message
...
On Fri, 20 Jan 2006 16:56:11 +0000, Serge Auckland wrote:
snip

When there was the 199? Football World Cup in France, I sold France
Telecom
a large number of automatic audio test meters so FT could test the 400+
ISDN
codecs they bought to provide audio feeds to the world's broadcasters.
These
ISDN codecs were being used on MP2 at 128kbps (pretty much as DAB is in
the
UK). Using spot frequencies, all tests were fine, as was audio quality
(mostly speech). but as soon as the automated sweep went through, the
frequency response had so many anomalies as to be meaningless. FT solemly
carried out the tests, failed every codec, but used them anyway....aren't
the French wonderful?........

Maybe they didn't plan on broadcasting frequency sweeps anyway.
:-)

/Martin

Quite right, but what amused me at the time is the way that they ran the
sweeps through all 400 odd codecs, and reported them all as having failed,
but clearly, as the sweep test was a nonsense, were happy to use them. It
made me wonder why they bothered to run a sweep test, but the spec said they
had to.........

S.