Re: Real Time MP3 Encoding

[Keith Doxey <keith.doxey@xxxxxxx>]

Hi James,

Thanks for the reply.
I want to record at a minimum of 32kHz (Broadcast Quality) possibly even
44.1kHz (CD quality). What I was trying to avoid was needing 600+ Mb per
hour for the temp.wav file.

The reason I want to keep a high sampling rate is that many of the programs
will contain music as well as speech and I dont want it to sound like an AM
station. Many radio stations are using quite low sampling rates for news
items and although my hearing isnt perfect by any means (too much time
working in nightclubs), I can hear a horrible high pitched whine from the
quantisation noise on the recordings.

I am not an audiophile by any means, just an engineer who cant stand noises
that shouldnt be there :-))

Encoding time isnt a problem as I will just let it run by itself overnight.


Having said that....any encoding that might have been running last night
would have been doomed to failure. Whilst sound asleep waiting for the
Homevision wake-up sequence that starts at 05:40 I was rudely awoked at
05:30 by the dishwasher blowing up !  :-(((

Not only did it burn out the wiring harness in the dishwasher door....it
also took the main 30Amp ring main fuse out with it. ;-((

Sometimes I really hate appliances. About 6 months ago the washing machine
died and we spent the whole of December without a grill waiting for a new
element to arrive !!

Keith Doxey
http://www.btinternet.com/~krazy.keith
Krazy Keith's World of DIY Home Automation


-----Original Message-----
From: James Derrick [mailto:james@xxxxxxx]
Sent: 30 January 2000 23:51
To: ukha_d@xxxxxxx
Subject: [ukha_d] Re: Real Time MP3 Encoding


On Fri, 21 Jan 2000 15:32:36 -0000, you wrote:

>Hi All,
>
>Whilst we are on the subject of MP3 Jukebox functionality a quick
question.
<snip>

>if I want to "tape" a 60
>minute radio program it would take over 600Mb of disk space. By
encoding it
>on the fly direct from the soundcard without an intermediate wav file
it
>will only take about 60Mb
Well, Keith, I've been doing just that for quite some time on my Linux
server to record Radio 4's "Science at 9:00" programs.

I have a script that records the audio as a WAV and then encodes it to
MP3 for serving to a client machine via NFS or Samba. This gives me a
'library' of science programs to listen to when I catch up with the
paperwork.

This approach gives give a couple of minor problems. All the MP3
encoders I've seen need a high bitrate (44k1- 32k samples/s) WAV file
to encode. You can't encode at 'radio-quality' and convert to MP3.
This means you need a bit of temp space. The encoding process then
takes a few hours so you can't really listen to a program that has
just finished using MP3 (although you could kill the encoder and hear
the WAV).

I did try a few experiments before adopting this approach and found
MP3 to be the best tradeoff of space and sound. I tried gzip'ed WAV
files which turn out to be actually a bit smaller than MP3 (6.8Mb for
30mins 8kHz mono PCM verses 7.8Mb 32kbps MP3), however these need to
be uncompressed on the client PC to be heard. WAV also sounds worse at
low bitrates with constant hiss, although music suffers from bad
frequency distortion under MP3.

I don't have stats for the MP3 encoding time as this just runs as a
batch job and isn't really noticeable. I use a PI-233 and BladeEnc-
the encode time is probably about 4x the length of the recording.

James
---
James Derrick    james@xxxxxxx, Cramlington, Near Newcastle,
England
Forwarding Service: jderrick@xxxxxxx
Beyond the Horizon of the place we lived when we were
young,
In a World of Magnets and Miracles. Pink Floyd.

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