ETDs, PCs, PDAs & cellphones vs tuning fork : how accurate are they ?

[Robert Scott]

If I may dare to jump into the discussion with Philippe and Geoff, let me say that for five years now I have been recommending that TuneLab customers use the NIST tones through their telephone to calibrate their Pocket PCs and laptops.  Throughout all that time, no one has ever reported to me that when they check the calibration using their cellphone that the pitch is anything other than rock-solid and repeatable.  If there were any day-to-day pitch changes due to something that happens through the cellphone network, someone would have noticed it by now.  I am reasonably sure that cellphones produce faithful representations of the pitch that was sent.  Precise time-base synchronization is the bread-and-butter of the cellphone companies.  They use it extensively to synchronize all the data transmission their network.  If there is a phase-locked-loop in the cellphone, you can bet that it has no audible frequency modulation.

However, because the question came up a few days ago, I am still investigating VoIP (Voice Over Internet Protocol) - the service that uses the Internet to place telephone calls.  Because of the lack of precise synchronization over the Internet, it is hard to see how pitch precision can be transferred over the Internet.  VoIP was one of the topics on last Friday's "Science Friday" on NPR.  After the program I e-mailed the the guest (the author of a book on VoIP) asking about possible pitch distortion through VoIP.  Unfortunately, this "expert" was actually unaware that audio CDs can have pitch distortion, depending on the precision of the playback speed.  So I was unable to get any useful information from him.  However, if any of you have VoIP, you can perform the following easy experiment and report back to the rest of us:

Call the NIST at (303)499-7111 in Colorado on your VoIP connection and simultaneously on your regular phone.  Listen for any beat between the two sounds.  There should be none.  But if there is, then we have evidence of pitch distortion through VoIP.

Robert Scott
Ypsilanti, Michigan