Frequency range of 'preFM' King Biscuit quad reels

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ssully

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This came up in a discussion I'm having offline about the King Biscuit quad (SQ encoded) aka 'preFM' reels that are often discussed here.

These (matrixed half-track 2 channel) reels must originate from well *before* the loss of King Biscuits master stereo reels in a building fire. Anything 'official' generated from those shows after that (e.g. the stereo mixes heard on Wolfgang's Vault since 2006 or so, and any CD releases from the KB vault) are 'new', nonencoded stereo remixes from multitracks (which would seem to still exist?!) .

As I understand it, back in the 70s a quad encoded halftrack/2-channel master mix was created by King Biscuit from concert multitracks, and duplicate encoded reels were then sent to various radio stations for broadcast. Later some of these 'preFM' encoded reels ended up in collector's hands, and are the claimed source of 'preFM' digital quad collectibles.

However some so called 'preFM' decode quad files I have in my collection show a clear cutoff at 15kHz. Others do not have this cutoff. Am I right to assume that the ones with the cutoff aren't really from preFM reels, but rather from broadcasts taped off the air? Would these show a very sharp 15kHz cutoff like I see?
 
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Plan9

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A sharp 15kHz cut-off would certainly seem to indicate a FM broadcast. I don't know why they would apply such a cut-off before broadcasting.
 

ssully

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So far I see the very sharp cutoff on just one KBFH quad show -- Yes , Dec 11 1974. I have decodes of this from two sources, done by two different people, and both have the cutoff
ODs' 'KBFH Live In Concert" (DS221f) , and another by an anonymous decoder, '75 King Biscuit Quadraphonic". Both purport to be from 'preFM' though the anonymous decoder expresses some uncertainty about that.

(this cutoff is also present on both stereo versions I have of this show )
Samples of the current Wolfgang's vault (stereo) version have no sharp cutoff, but also haven't much content above 15. Their spectrum looks...weird.

These other quad radio show decodes don't show a really sharp 15 khz cutoff, though some of them show only a little content above that
King Crimson May 26 1974 - KBFH 26-5-74 (DS321f)
ELP Feb 2 1974 - KBFH - Live In Concert (DS212f)
Genesis Aug 16 1976 - Hammersmith (British Biscuit) (BTN-015) "Tape provided courtesy of the GRASP Genesis group"
ELP Feb 2 1974 - KBFH - Live In Concert (DS212p) 'decoded from SQ pre-FM' -- extremely little above 15
Genesis Apr 15 1975 - Empire Pool Wembley 15th April 1975 (DS99) -- not much above 15 but no sharp cutoff


Curiously, the stereo version of ELP Feb 2 174 I recorded off of Wolfgang's vault years ago shows the very sharp 15kHz cutoff. But the current version on the Vault page, of same show, is not frequency limited.
 

steelydave

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I think the waters on this get muddied by a couple of things - one is the terminology bootleg traders use for tape recordings of radio broadcasts (ie guy in his house recording something off his own radio using a cassette deck0 which is "off-air master." To me (anyway) this is kind of confusing, as I've always thought of radio brodcasts as "on air" or "on the air" so I used to wonder does 'off air' mean a pre-broadcast tape? But it's my understanding it doesn't, it just refers to (as I said) stuff taped off a radio broadcast. So there's a chance that people are either accidentally misconstruing 'off-air master' to mean pre-FM master, or possibly intentionally labelling things as 'pre-FM' (even though they know they aren't) simply to make them more popular or desirable amongst whatever circle of collectors they run in.

The other possibility is that some of these recordings have been through a low-quality mp3 generation somewhere along the line, which happens quite often in bootleg circles. I know (for example) there's at least one highly desirable Black Sabbath bootleg where every version stems from an mp3 source, and nearly 10 years after it first surfaced no one has been able to find a version that doesn't have this lossy step in its lineage. I believe 128kbps mp3 introduces a hard cut around 16kHz to save data, and I wouldn't be surprised if other lossy formats (aac, ogg, wma, etc.) employ a similar scheme.
 

ssully

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To me, 'off the 'air' in this context always meant the source was made by a listener at home (i.e., recorded on cassette decks or consumer RTR decks -- or captured digitally if the broadcast is from the last few decades). I expect these to have no useful content above 15 kHz, the upper band limit of broadcast FM in America. (I'm not sure how 'sharp' the cutoff should look, though...the very few cassette tapes of FM broadcasts that I digitized ages ago, with probably added EQ and incorrect Dolby decoding, show a fuzzy cutoff at 15-16khz with tape noise above that, not the sharp line and lack of noise I see in some 'preFM' in circulation.)

"PreFM" meant, the source was a reel sent to a radio station for broadcast.. the best possible source short of the actual original mixdown master reel created by King Biscuit (the one duplicated for distribution to stations) . I would not *expect* these to be aggressively bandlimited @ ~15kHz....but I keep encountering (moreso in the stereo boot world, so far) supposed 'PreFM sourced' files that are just that. So either I'm totally wrong about the bandlimiting of preFM reels, or a lot of traders/seeders are confused/misinformed (or someone just plain lied).

15kHz vs 16kHz is certainly something that could be checked.
 
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ssully

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My offline friend is arguing that KB's distribution reels sometimes may have been purposely bandwidth limited (e.g. to thwart bootleggers). I consider that implausible (no bootlegger would care, especially in the days of vinyl)) but I can't rule it out.

Does anyone really know one way or the other?
 

jimfisheye

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Some roio traders will misuse terms to denote sound quality. Not maliciously but misinformed. (Although the former happens often enough too.) They hear a better sounding audience recording and call it a soundboard, for just one example.

There are at least two wildly different transgressions that can low pass at 15 or 16k - FM broadcast and lossy audio data compression as mentioned. It's really a hard low pass too! I've tried digging down into the decimal dust in some of these trying to do restoration and there really isn't anything left up there.

It's usually happiness and light when you find an original vinyl transcription disc from the late 1960s. They were all intended to be returned to be smashed and the vinyl returned for recycling.

These KBFH recording are hard to collect if you're not in some inside circles! I figured the lossy mp3 downloads and the chirpy hyped sounding CDs (that all have that roio vibe) were more of the "Look at what I have but you only get a stepped on copy of" kind of thing. And here we are years later with the originals buried out of sight and mind.
 

ssully

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"PreFM' KBFH reels (or purported copies thereof) show up on ebay with some frequency. There's one (stereo, not quad) up there right now:


"THIS TAPE IS A COPY FROM A RADIO STATION TAPE, IT'S NOT AN ORIGINAL"

Shows distributed on CD show up more often:

 
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Owen Smith

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If the tape is a copy not an original, why would anyone want it on tape? Surely a digital copy of the original would be much more useful and desirable.
 

MidiMagic

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I serviced tape machines in the 1960s and 1970s. And in some cases, I found that the recording bias frequency was too low to allow what the industry was touting as 20Hz - 20KHz. I found many machines with bias frequencies from 16KHz up to over 25 KHz.

Up to the mid 1960s, the standard frequency range (and that of FM radio) was 30 Hz - 15KHz.

FM has the stereo pilot tone at 19 KHz, so nothing could be sent nearly that high in audio frequency.
 
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Soundfield

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If these tapes were 71/2 ips recordings I’d be amazed if there wasn’t a sharp cut off around 15kHz. The frequency response of tape falls rapidly with reduced tape speed and track width. These are the plots for the legendary 1978 professional Studer B67 half-track machine-
tape freq response.JPG



and the drop off above 18kHz at 71/2ips is very sharp. It would be much worse for any lesser or semi-professional machine of 1970’s vintage let alone any domestic deck.
 

par4ken

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If the tape is a copy not an original, why would anyone want it on tape? Surely a digital copy of the original would be much more useful and desirable.
Make your own digital copy. The tape might of been from someone's personal collection dubbed from the original many years ago! I wouldn't pay that kind of money for a taped copy though!
 
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par4ken

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If these tapes were 71/2 ips recordings I’d be amazed if there wasn’t a sharp cut off around 15kHz. The frequency response of tape falls rapidly with reduced tape speed and track width. These are the plots for the legendary 1978 professional Studer B67 half-track machine-
View attachment 74004


and the drop off above 18kHz at 71/2ips is very sharp. It would be much worse for any lesser or semi-professional machine of 1970’s vintage let alone any domestic deck.
The drop off is only -1.25 dB @ 20KHz, barely noticeable, hardly a sharp drop off!
 

marcb

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If the tape is a copy not an original, why would anyone want it on tape? Surely a digital copy of the original would be much more useful and desirable.
Both are still dubs. It depends upon how good or bad the dub of each was done.

But more than likely it’s as simple as someone likes to collect reels.
 

ssully

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If these tapes were 71/2 ips recordings I’d be amazed if there wasn’t a sharp cut off around 15kHz. The frequency response of tape falls rapidly with reduced tape speed and track width. These are the plots for the legendary 1978 professional Studer B67 half-track machine-

and the drop off above 18kHz at 71/2ips is very sharp. It would be much worse for any lesser or semi-professional machine of 1970’s vintage let alone any domestic deck.

I see rising response starting at 3 kHz maxing out at ~+1.2dB @ ~10-12kHz, then falling off back to pre 3kHz level at ~17kHz. Not particularly sharp filtering.

Anyway, what I'd really like to see here are reports from others who have 'preFM' files in their collections. Throw a track up on Audacity or some other app with spectral view, and tell us (or post) what you see. Later today i'll post some screen caps of things I see.
 

furui_suterioo

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I see rising response starting at 3 kHz maxing out at ~+1.2dB @ ~10-12kHz, then falling off back to pre 3kHz level at ~17kHz. Not particularly sharp filtering.

Anyway, what I'd really like to see here are reports from others who have 'preFM' files in their collections. Throw a track up on Audacity or some other app with spectral view, and tell us (or post) what you see. Later today i'll post some screen caps of things I see.
Yes Boston Garden "Pre-FM" MP3 files, definitely looks like a cutoff around 16khz
20211201_135626.jpg
 

ssully

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Thanks! Can you identify exactly which release of this show, this is from? (There are a dozen versions out there)
 

ssully

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That is funny. Also kinda bogus. I have a copy of the true DS221f release from OD, and it's definitely 4.0 multichannel. The front cover artwork doesn't have the words 'ANALOGKID SHARES YES" on it, though. God knows how that one was made...a downmix from the true DS221f, then converted to mp3??

Or maybe it started from an mp3 in the first place.
Here's what a few loud seconds of the true 4.0 DS221's front left channel looks like in Audacity frequency plot. Sharpest filter is just above 15 kHz, and then another at 16.

1638397834522.png


and here's spectral view of a bit of the front left channel. I used the same settings as for the graph, but the color gain is boosted 20dB by default in audacity, and I increased contrast further, so faint signal is visible up to and beyond 17kHz .
There actually seems to be *three* 'steps' in this filtered audio
- just above 15 kHz
- at 16 kHz
-just below 17 kHz

all the faint spiking over 17 is perhaps artefact of digital processing.

(Not shown: level starts to drop off at the low end at 50Hz. )
.
1638398201683.png
 
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