16 KHz stripe in Master Open Reel Recording

[guygee]

Trying to help somebody with this conundrum. The FA and SAs below are supposed to be from a house recording master open reel 7.5 ips (unknown deck). The transfer was done "professionally", so no details are known.
The job is to track, flac, organize and seed. Some of the previous seeds are running here now.

What boggles my mind on this recording is the 16 KHz stripe. This recording does not look like a broadcast FM to me...way too much good frequency content above 16 KHz. So why the 16 KHz spike/stripe?

I am speculating that the 16 KHz tone was injected for pitch control (playback speed control) during taping, and this tape was prepared as a pre-FM, but I would very much appreciate anyone else's informed opinion. Has anyone seen this type of thing before?

Thanks!

[U2Lynne]

I seem to recall Five talking about getting this sort of stripe on some transfers he did and he wasn't sure why they showed up. He's off on vacation right now, but I think he's supposed to be back next week. Maybe he will have some insite.

[direwolf-pgh]

Carrier wave/Frequency modulation

http://en.wikipedia.org/wiki/Carrier_wave
http://en.wikipedia.org/wiki/Frequency_modulation

thats my swag

[paddington]

you guys are thinking about the 19kHz stereo 'pilot' tone in FM radio. This isn't that, unless the tape is running VERY slow.. which isn't likely.

I'd say that is line noise - electrical interference on the audio cables during recording - or playback.

We'd need to know how it was transferred to WAV to be able to guess about that. If you can post audio samples of some quiet places in the recording, I can analyze it and tell you where it probably came from.

[guygee]

Thanks Lynne and direwolf for taking a look. Jameskg, there were a couple of small tracks marked silence, I guess from the tape lead-ins/lead-outs that have no magnetic material on them. These were truly silent, with nothing much in them at all. I guess that rules out problems on playback.

I don't think it is the pilot tone either, but your idea is that during the recording the cables could have been acting like antennas and picking up on some interference? Unfortunately I looked through the whole show, and besides those lead-in/lead-out tracks not a silent place anywhere to check...crowd noise everywhere in the background, and the same ~16 KHz stripe.

I guess the main thing I wanted to do is rule out FM broadcast...these are supposed to be from the masters...but I am still sure damn curious about that stripe.

Not having the original tapes makes this pretty hard to figure out.

[Audioarchivist]

I recently transferred some King Biscuit Flower Hour vinyls (ZZ Top Passaic NJ 1980) that have a 16000 Hz stripe in them, too. I don't get that for any other transfers I've done, from vinyl or tape or anything. It dissappears during silence at end of sides, but is embedded in the music. I first thought it was interference (like a tv being on in the next room or something, but evidently not.
Since these albums were pressed specifically for radio broadcast, I'm thinking DIR Broadcasting put the signal in to the vinyl to carry over the radio better.
So, it doesn't mean that having a 16kHz stripe means it's POST-fm necessarily. My pre-fm vinyl source is what they played over fm. It's obviously been prepared for radio airplay when it was mastered and pressed. It's not just a mark of the radio station's transmitter...

[guygee]

Thank you Audioarchivist, that is exactly the kind of information I am looking for. Some people claim these is no difference between a Pre-FM and a master recording (be it sbd or say mics onstage), but it seems this recording was also prepared as a Pre-FM, with the tone at about 16 KHz actually injected into the master. Many shows from this venue ended up being broadcast on FM, so it all begins to make sense.

I was looking though some old posts, and in this post ssamadhi97 identified a Westwood One promo CD (Toad The Wet Sprocket1990-04-11 Live at Park West) with the same stripe. Another true Pre-FM(?).

I would love to know "why" they put that signal into a pre-fm? Is it getting used for something, like pitch correction on playback over the radio? Or something else?

[Audioarchivist]

I believe it's put there to enhance the way fm signals tend to deplete the highs.
I help with a pirate radio station, and we have to eq up highs and cut bass frequencies to unusual amounts to broadcast a flat response over the air...
The 16kHz stripe I guess lets high freqency signals cut through better by sort of fooling the transmitter/receiver combo a little bit...
Having that high signal present opens up the radio waves to better let other real audio signals piggyback on top of it.

[paddington]

Let me stop you guys from rolling down the wrong track with this FM radio and 16kHz thing. What Audioarchivist is thinking about in terms of the high frequency help in FM transmission is a pre-emphasis curve of 75uS (microseconds) - which raises the modulation of the higher frequencies in a logarithmic curve that is then de-emphasised in the receivers. This allows a better S/N ratio over the entire system to combat the noise that is naturally more prevalent in the higher end of the audio spectrum.

If you record from the receiver, it should already be de-emphasised, and you wouldn't see much in the analysis to betray the presence of the processing (unless the receiver is a piece of shit). FM transmission has a response of 15kHz on each channel and rolls off sharply after that. That doesn't mean you would see a tone at 16kHz. I don't know where people get that from.

Now, you may still see the 19kHz stereo pilot tone. This tone tells the receiver to 'de-mux' the audio it gets to 2 stereo channels using the info at the second harmonic up from the pilot (38kHz). This is done to allow the same FM transmission to be received on both mono and stereo receivers. The L-R signal, or 'difference' signal is applied to the sum signal (L+R) and you get "stereo separation" to whatever degree the difference (L-R) signal is applied. In most cases, you get about 20% or so of the L-R signal. The people at Dolby stole this method when they created the original 4 channel Sourround Sound, for the back channel. It's the L-R difference signal in the mux.


Anyway.. the 16kHz noise needs to be analyzed before we can decide what it is. Don't assume it's a tone (which assumes a sine wave) - I'm betting it's white noise.

When I say "quiet" spot, I don't mean "silent" - any quiet passages in the music will be fine. The reason I ask for quiet is that the noise is likely a constant level, so any time the program audio is softer, the noise will be better isolated. If you can post some samples, I'll tell you what it is.

[direwolf-pgh]

EMI/RFI

http://en.wikipedia.org/wiki/Radio_f...y_interference
http://en.wikipedia.org/wiki/Electromagnetic_radiation

http://www.hydrogenaudio.org/forums/...://t17212.html

There are three distinct signatures in the sources used in the original source. Each can be seen clearly in spectral analysis. A 16khz spike can be seen in two of them, one having a lower B/W of approximately 10khz, and the other have a B/W of appoximately 16khz. The third has the same 16khz B/W but without the 16khz spike. The 16khz spike is, likely, due to CRT Horizontal EMI during the recording process.

http://db.etree.org/shninfo_detail.php?shnid=32875

[guygee]

Many thanks you guys for trying to help with this. Here is a 1.5 second stretch from a quiet part. The crowd is silent and only the electric guitar is playing a very soft riff. jameskg, I think you are right, that line looks a little too hefty for a simple sine wave. Please let me know what you think.

[guygee]

Originally Posted by jameskg

If you record from the receiver, it should already be de-emphasised, and you wouldn't see much in the analysis to betray the presence of the processing (unless the receiver is a piece of shit). FM transmission has a response of 15kHz on each channel and rolls off sharply after that. That doesn't mean you would see a tone at 16kHz. I don't know where people get that from.

Quite apart from the issues of this recording, I've looked at FAs and SAs of many FM recordings made at the receiver, and it seems this 16 KHz signal (actually just below 16 KHz) is very often present. I've researched this topic, and found specs for modern hi-power FM receivers, and one of the features commonly mentioned is sampling at 32 KHz. Going back into the '70's and '80's I would guess that the technology was different but the effects were nearly the same (using analog filtering instead of digital sampling). Regardless, it seems this signal often survives to make it into a recording made at the receiver. ( A lot of crappy FM receivers? I wouldn't be surprised if that is the case. Or maybe many people have their TVs on while recording...thanks for that interesting angle direwolf.)

[guygee]

One last screenshot SA - crowd goes almost silent during count-in to the song (...2, 3, fouh).

[Audioarchivist]

I re-checked my stripe-y example, my vinyl transfer of a King Biscuit Flower Hour lp of ZZ Top.
The stripe is there sometimes and is gone other times...
There's commercials on the record, and the stripe dissappears during those breaks, and re-appears for some songs, but by the end of the show it's gone for good. I can conclude that it wasn't added on my equipment, but is embedded in DIR broadcasting's master tape.
It's in the show for side 1 and 2 but gone for side 3, and it's not in the commercials.
The first two caps are from side 1 and 2, the third is nearer the end of the show.
As I looked through the whole show, like I say, it goes away when the music fades and a commercial on the vinyl plays, so it can't be interference from my monitor or tv(which wasn't on during turntable operation, anyway)...

[paddington]

16kHz stripe doesn't come from King Biscuit. I've transfered those myself from virgin broadcast vinyl (a couple are still posted at this site) and that stripe isn't there. It's possible that the person playing those records had the NRSC pre-emph versions and didn't know how to de-emph them.

I still say it comes from a bad transfer. People use bad cables. TVs and monitors with CRTs in them emit a 15,750Hz noise. That's is common knowlegde among people who build recording studios. You must keep your audio WAY away from them. That noise shows up all the time in transfers done by hobbyists with home computers.

The stripe coming and going could just be the environment changing during transfer. The stripe on the ones you just posted is at about 17.2kHz. If it was some standard thing, they wouldn't be all over the place. Check the SA on my Plant Biscuit torrent that is here... it's straight from broadcast vinyl transferred on pro gear. No noise at 16kHz or anywhere else.


Guygee, what I meant by sample was an audio sample ... I'd like to have a minute or so of audio in those quiet spots to work with. Sorry for the misunderstanding