Real time recording

[BosseH]

When I am recording analogoue audio from a radio receiver I set the start of recording to my actual time (start marking).
After 8 hours of continuous recording the Audacity marking position has speeded +25minutes compared to real time. How come?
The time marking must be able to measure exact timing otherwise it is useless. Is there something I can do to get Accurate time recording?
I would like Audacity to have built in real time clock to be able to be set by the individual user depending on time zone.
Looking forward to your comments.

[kozikowski]

"Time" in a computer is slippery. Audio recordings are made with reference to the digital clock system on the sound card and they're played back against two clocks, the one in the computer and the one in the sound card -- which may be different from the recording clock.

All of these clock chips/electronics together may have cost three cents, tops. Nobody is expecting world class accuracy from these systems.

That and we admonish people that Audacity is not a surveillance system and really messy things happen if you try to make it into one. How many recordings have you made and has Audacity started to drop data or freeze yet?

Koz

[steve]

Also note that the maximum file size for a WAV file is 4GB http://en.wikipedia.org/wiki/WAV#Limitations

[BosseH]

Thanks for your comments. I have done just 3-4 test recordings up to 10 hours length. I use sampling rate 8000Hz and this means the largest file so far, 10 hours is about 1.2 Gbyte. All recordings are speeding about 2-minutes/hour related to real time. I did not know there are several clocks involved so I can understand this is a tricky item. I am not a software guy. I am a DX-er (shortwave, mediumwave enthusiast) and I can see a great benefit of using Audacity
for recording radio stations especially if the recording could be matched to real time, for instance UTC. I thougt it might be possible to sync Audacity to the
main computer clock. Obviously this is more complicated than I realized. No signs of Audacity dropping data or freezing.
More comments are welcome.

[steve]

Why are you using a low sample rate for the recording?

[BosseH]

I get god enough audio from the radio recording at sample rate 8000Hz. The low sample rate creates much smaller files in .aup format.
I have a small harddisc im my computer. By changing from 44100Hz to 8000Hz the recording space on my harddisc (20Gbyte free space) increases from about totally 20 hours to totally 170 hours!

[steve]

The reason that I ask is that the most likely cause of the inaccurate time is the sound-card clock signal is inaccurate. You may find that it runs a bit more accurate at different sample rates. It may be worth trying it at 11025 Hz to see if it is any more accurate.

[avalon]

One option would be to change speed a little bit after recording.

Does your card support 8kHz natively? The cards I've seen, do not. You may try recording at a higher sample rate and then down sample.

And last you may try recording with arecord or sox with a higher samplerate. Pipe the output to an speex/ogg/mp3 encoder and you get small size/better quality.

[BosseH]

Thanks for tips. However the only thing I want to achieve is a simple way to record analogue audio and use the Audacity time counter as reference to real time.
At playback I want to see at which time a certain tune/artist (for instance) was played.
I have now tested some different sampling fq:s : 11025Hz : The Auacity time counter is LOSING~2 minutes/hour. Same thing at 16000Hz
22050Hz : The counter is losing about 5 minutes/hour. Same thing at 32000Hz
New test at 8000Hz : the counter is speeding about 1 minute/ hour.
So the counter result seem to be more or less random. If it depends on the sound card on my laptop (Dell Latitude D620) there is not much more to do.
Finally: Is there no possibility that the Audacity time counter software could be synced with the computer real time clock?

[steve]

If it depends on the sound card on my laptop (Dell Latitude D620) there is not much more to do.

Other than upgrade the sound card.
I have an inexpensive USB sound card that I use with my laptop (because the sound quality is much better than the on-board sound - particularly when recording). I've not tested the time accuracy on long recordings as most of my recordings are quite short, but I'll try to remember to do that and report back the findings.

Is there no possibility that the Audacity time counter software could be synced with the computer real time clock?

If you mean the "time line" (the horizontal rule above the audio tracks) then no, it's tied to audio data.
If you are recording at 8000 Hz, then Audacity assumes that there are 8000 samples each second. You can check this by selecting exactly 1 second of audio (enable "Snap to" and set the time units in the Audacity "Selection Toolbar" at the bottom to "seconds"), then change the time units to "samples" and there will be exactly 8000 samples.
The time problem is if your sound card gives Audacity 8003 samples per second, or 7999 samples per second when it should give exactly 8000 per second. The reverse problem is on playback - when you play a track that has a sample rate of 8000 Hz, then the sound card should play exactly 8000 samples each second. If the sound card actually plays more or less than 8000 samples per second, then playback will be either a bit too fast or a bit too slow.