On 4/23/99 12:05, Leigh S. Jones at kr6x@kr6x.com wrote:
>For a 30 hour contest you would need ~10 Gbytes of storage if you used 16
>bit audio with CD quality settings. This suggests using lower quality
>audio settings (8-bits and low rep rate) and compression such as u-law.
Well, let's think about this a second. First of all, communications audio
is roughly 300-3000 Hz. There's no reason, therefore, to sample at a rate
much higher than about 6 kHz, since that extra bandwidth isn't used
anyway.
Secondly, the dynamic range of typical communications audio isn't as deep
as telephone audio, so you can get away with fewer bits per sample.
It would be safe to say that if you used the telephone standard of 8 kHz
sampling, 14-bits/sample compandered to 8-bits with u-law encoding, you
can easily store the entire contest period on the hard drive of modern
computers.
Even if you do sample at CD audio rates, you can do a rate conversion to
limit this to a much lower sample rate. One simple technique is just to
throw away 3 of every four samples. That gets you down to 11.025 kHz at
16-bits/sample.
>This type of file cannot be reduced in size by pkzip.exe, ntfs, etc., due
>to the randomness of the data.
I wouldn't say it won't compress, but it might not compress as well as
say, a text file. If you have long periods of virtually zero samples, it
might compress quite well with these lossless compression techniques.
>If your sound card didn't come with
>software that will continuously stream data to disk for an hour or so,
>then automatically start a new file name, you will have to write your own
>program to do it, because there is not much of a market for this type of
>software.
Isn't this built into the Windows OS? The Macintosh Sound Manager has a
single call to do exactly this -- record directly to disk.
>You probably
>won't want to create the whole thing in one file -- it could exceed the
>maximum file size that you are capable of handling.
Hmm. 11.025 kHz * 2 bytes/sample is 22,050 bytes/sec or 21.5 KB/s.
An hour would be about 76 MB. 24 hours is 1.8 GB. Seems like a 48 hour
contest could fit without exceeding the file size of 4 GB. And you could
knock off some file size by using compandering and/or rate conversion
without affecting quality.
>A
>video tape recorder or DAT has been the method of choice for the informed
>contesters.
Yea, but computers are more and more capable these days (not to mention
having a lot more storage that they used to.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@radio.org
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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