Message-ID: <431D0943B8D44D878E6FC4E18C8A20BF@Extensa>
From: "PA1SDB, Peter" <pa1sdb@amsat.org>
To: <rsgb_lf_group@blacksheep.org>
References: <FE434AB14D2442DA9517B6F7304CE6E3@AGB><FB72BFCD7B564064B0C93A0FD2DA9448@Extensa><4F97E9C2.9030000@iup.uni-heidelberg.de> <CAA8k23REzyvnLSkQGwzh4ckSFTeSFSPQkUMD2z-K9-YTGZfxQw@mail.gmail.com> <7B23411450514A5CB804E6D2C65A1876@JimPC>
In-Reply-To: <7B23411450514A5CB804E6D2C65A1876@JimPC>
Date: Sun, 6 May 2012 15:39:38 -0000
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Subject: Re: LF: Generating 8970 Hz carrier with Spectrum Lab ?
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OK Jim,
Tnx for sharing your experiences...
73's, Peter PA1SDB



----- Original Message ----- 
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Sunday, May 06, 2012 3:14 PM
Subject: Re: LF: Generating 8970 Hz carrier with Spectrum Lab ?


> Dear Andy, LF Group,
> 
> A bit late, but never mind...
> 
>> Has anyone tried using an external USB soundcard with a separate
>> locked clock?   Most work from a 12MHz crystal which can be replaces
>> with a GPS locked source without too much effort. But I can't help
>> wondering if there will be subsequent USB synchronisation glitches
>> upsetting the input sampling.
> 
> 
> I can confirm that glitches do occur with USB sound cards. I have found this 
> to be a perennial problem trying to use such a sound card with the laptops I 
> have available. For 9kHz reception, the relatively rapid temperature 
> fluctuations inside the laptop, and resulting cyclic drift of the internal 
> soundcard sampling frequency interfere with the operation of DL4YHF's 
> ingenious sample rate correction facility in SpecLab, making the internal 
> sound card unusable for FFT resolution below a few millihertz. I found my 
> USB soundcard solved this particular problem quite well, but introduced 
> glitches that made achieving FFT resolution in the uHz range impractical.
> 
> Watching Speclab's sample rate correction "history" window, the USB card 
> sample rate typically starts off a few hundred ppm low (much larger than the 
> actual clock frequency error), but remaining stable to within a few ppm, but 
> then at unpredictable intervals abrupt jumps in sample rate of a similar 
> order of magnitude occur, with corresponding "blips" on the spectrogram 
> traces. The reported sample rate is always lower than the nominal value, 
> suggesting that some samples are being periodically discarded somehow.
> 
> The sound card uses a single-chip integrated audio codec and USB 
> transceiver, using a single 12MHz crystal. I can't really believe in "USB 
> slippage" in the hardware - surely losing some of the data would either be 
> handled quietly by the USB error checking, or result in endless error 
> messages. The same sound card seems to work in a glitch-free way when 
> plugged into my desktop machine, where the reported sample rate error is in 
> line with the error in the crystal frequency.
> 
> Cheers, Jim Moritz
> 73 de M0BMU 
> 
> 
>