What's the phone-like sound at the end of my video tapes?

July 10, 2006 10:11 PM

So I've been watching a bunch of older movies on VHS like Wait Until Dark (1963) and Sword in the Stone (1967). At the end when everything's black, right before it turns to the black and white noise, there's a sound of a phone dialing very quickly, or something similar. Anybody else ever noticed it? Out of curiosity, any ideas on what that is, how it happens, why it's there, or anything at all about it?
posted by freddymungo to grab bag (17 comments total)

I think what you are referring to is macrovision, an attempt to thwart duplication.
posted by AllesKlar at 10:18 PM on July 10, 2006

Is this a quick little series of rising tones? I've noticed the same thing on some cassettes. I presume it's a sort of test-pattern—frequency response of your audio channel can be measured by what portion of the pattern is audible, or such.
posted by cortex at 10:20 PM on July 10, 2006

If it's the same thing I've heard, it's a sequence of tones, about four tones per second, with each tone an octave above the one before. I'm guessing that this is the cue tone referred to about half way down this page.
posted by flabdablet at 10:50 PM on July 10, 2006

I have heard it on VHS tapes. It can't be macrovision because it only plays for a few seconds. You could just fast forward past it.

Remember, DTMF tones are very easy for electronic devices to detect, that's why they were used in the phone system (way before computers).
posted by delmoi at 12:01 AM on July 11, 2006

Oh, from here (googling "VHS DTMF")

It's called DTMF signaling. Its information that is put on the master video that tells the automated duplication equipment essential information about what is being recorded. Information ranging from audio levels to the overall time length of the video is encoded into those tones. When the individual recorders receive the tones, it will automatically make the necessary adjustments to ensure the highest quality video possible.
posted by delmoi at 1:20 AM on July 11, 2006

I've heard similar DTMF tones on mass-produced audio cassette tapes, as well.
posted by neckro23 at 1:50 AM on July 11, 2006

When we first got a VCR those tones fascinated me. Thanks for asking.
posted by sohcahtoa at 4:22 AM on July 11, 2006

Yes, thanks for asking. I've heard that countless times, kept forgetting to research what it was, and somewhere along the line started taking the tones for granted.
posted by Tuwa at 5:31 AM on July 11, 2006

It looks like it actually is the sound of a phone dialing; I guess videos apply the same principle to provide different information. Cool how that works; thanks for the answers.
posted by freddymungo at 7:01 AM on July 11, 2006

I remember hearing it during cable TV in the 80's. Especially on the Nickelodeon network.

I still hear it on OLN. In fact, I heard it last week.

I suspect it's some kind of signaling tones, but I don't know what they'd be signaling.
posted by Wild_Eep at 7:13 AM on July 11, 2006

If you're hearing in on a *broadcast*, then it usually *is* DTMF tones, and it's an in-band signal to an early broadcast commercial insertion system to insert local advertising in a break -- we had such an encoder in the rack when I worked at MOR Music TV in the early nineties.

These days, I think they do that in a fashion which isn't audible.

It wasn't *supposed* to be audible *then* -- there was a delay long enough for the DTMF decoder to lock up that was supposed to be put in the audio path, but not all systems were lined up properly.

If the VHS tape was *not* made off-air, I'm not sure what it would be, and if the tones were *not* DTMF tones (which "4 notes each rising an octave" does *not* sound like), they might well be audio line-up tones for checking frequency response on the dubbing rack.
posted by baylink at 8:15 AM on July 11, 2006

I hear DTMF signals (I guess) all the time on CourtTV. Drives me friggin' nuts. Between the show and commercials, it sounds like a phone call, between commercials it's in there sometimes, and then from the commercial to the show. Beep-boop-bop-beep. Ugh.
posted by ajpresto at 8:46 AM on July 11, 2006

If it is this:
It's called DTMF signaling. Its information that is put on the master video that tells the automated duplication equipment essential information about what is being recorded. Information ranging from audio levels to the overall time length of the video is encoded into those tones. When the individual recorders receive the tones, it will automatically make the necessary adjustments to ensure the highest quality video possible.

shouldn't it be at the beginning rather than the end of the program being recorded?
posted by Flashman at 10:51 AM on July 11, 2006

shouldn't it be at the beginning rather than the end of the program being recorded?

Perhaps it's a signal to the recording equipment that tells it that there is no more tape to copy.
posted by lockle at 11:55 AM on July 11, 2006

I agree that it's an "end" signal. The reason for using a multiple-step DTMF is to minimize the chance of false-positives from the film soundtrack.
posted by Steven C. Den Beste at 1:48 PM on July 11, 2006

One of my very favorite musical artists (ok, he's my brother, but still...) made an instrumental song out of the ones that are on audiotapes. You can listen to it here if you want to see if that's the same thing as a DTMF. (I guess now that people don't grow up hearing these sounds on their tapes, the song's basis is becoming obscure -- makes me feel old!)
posted by daisyace at 6:23 PM on July 11, 2006

The tones I was talking about earlier are the ones that daisyace's brother used for his main riff. There seems to be a bit of DTMF back there in the rhythm track too.
posted by flabdablet at 4:49 PM on July 12, 2006