Topic: Does anybody know what this is??

[TheJez]

Hi all,
A little while ago I took my two recorders (Edirol R09HR, Tascam FR-AV2) and two sets of mics (Clippy's, Core Sound Stealthy Cardioids) to a local gig for some testing. (It was my first outing with my new FR-AV2 and I didn'tcare about the gig itself.)
I recorded parts with Edirol + Clippy's, Tascam + CS, Edirol + CS and Tascam + Clippy's. The Edirol recorded at 96kHz, the Tascam at 48kHz. After opening the recordings in Audition, I noticed on all the recordings that there was a weird peak in the 'frequency analysis window', see attachment. The peak is rather constant and seems to be at ~22kHz. As it is present on all four combinations of recorders and mics, I guess it is not something caused by one of the recorders or mics. The frequency remarkably resembles half of the CD-standard 44.1kHz sample rate, so I thought it must have been something produced by some equipment with poor D/A converters running at 44.1kHz, through the PA...

But then, a few days ago, I recorded a gig that I did care about at a different, well-known venue. I used the CS and Tascam to do so. The recording came out fine, but... there was again this peak at ~22kHz in the frequency window! However, this time it was not constant. Instead, it was there for 3 seconds, then gone for seven seconds, then again for 3 seconds etc etc. Very intriguing! This was the case for both support act as main act.

My question to you, fellow tapers, is this: Have any of you ever seen something like this? And do you have any idea what this might be?

Best regards, Thanks for sharing your thoughts and opinions!

[Ronmac]

It looks like the 22Khz pilot tone used in some distributed speaker systems to monitor line integrity.

[TheJez]

It looks like the 22Khz pilot tone used in some distributed speaker systems to monitor line integrity.

Thanks! Interesting and you may be right. I'd never heard of such a system before, and I'm not sure if I understand the use case for it in concert venues. I mean, if sound is coming from the PA, the line is okay. If no sound, then it's not okay... Don't need a 22kHz pilot tone for that???
Hmm, okay, with this inaudible tone the line can be checked without producing audible sound. I guess my guide dog won't like it, though 

[morst]

I never knew about the 22k Pilot Tone but it appears to be a THING! I bet they have a "scheduled maintenance check" for three sec every ten sec... see below

Distributed Speaker systems are not what's usually used for rock and roll audio.
But if these venues have house systems with integrated controllers, the pilot tone could help diagnose those in real time.

Check this box out
https://www.audiologic.co.uk/partners-area/products/pilot-tone-load

The PTL-1 is a Q-Sys accessory that provides an end-of-line 22 kHz impedance soak for 70/100V loudspeaker circuits. Used in combination with a QSC DataPort-equipped amplifier or CXD platform amplifier, the PTL-1 becomes a vital component for monitoring of a distributed loudspeaker line

Key features
Within Q-SYS, a 22 kHz pilot tone (outside the audible range of human hearing and undetectable above the program material) is transmitted along each loudspeaker line.
Q-SYS, knowing the impedance value of the PTL-1, is then able to easily detect any deviation in the impedance at 22 kHz.
Any significant drop below a predetermined impedance will signify that a short circuit has occurred, while any rise above signifies that an open circuit has occurred. Each failure scenario will then generate a time stamped event into the Q-SYS Event Log,
Additionally, the user can select whether the pilot tone is only generated during a scheduled maintenance check, or whether the tone is always active to provide real-time diagnostics on every zone.

[Gutbucket]

[TheJez]

I never knew about the 22k Pilot Tone but it appears to be a THING! I bet they have a "scheduled maintenance check" for three sec every ten sec... see below

Distributed Speaker systems are not what's usually used for rock and roll audio.
But if these venues have house systems with integrated controllers, the pilot tone could help diagnose those in real time.

Check this box out
https://www.audiologic.co.uk/partners-area/products/pilot-tone-load

The PTL-1 is a Q-Sys accessory that provides an end-of-line 22 kHz impedance soak for 70/100V loudspeaker circuits. Used in combination with a QSC DataPort-equipped amplifier or CXD platform amplifier, the PTL-1 becomes a vital component for monitoring of a distributed loudspeaker line

Key features
Within Q-SYS, a 22 kHz pilot tone (outside the audible range of human hearing and undetectable above the program material) is transmitted along each loudspeaker line.
Q-SYS, knowing the impedance value of the PTL-1, is then able to easily detect any deviation in the impedance at 22 kHz.
Any significant drop below a predetermined impedance will signify that a short circuit has occurred, while any rise above signifies that an open circuit has occurred. Each failure scenario will then generate a time stamped event into the Q-SYS Event Log,
Additionally, the user can select whether the pilot tone is only generated during a scheduled maintenance check, or whether the tone is always active to provide real-time diagnostics on every zone.

Great, great! Mystery solved and learned a new thing. If I ever get to speak to sound tech people at these two venues, I will ask them about this! Also will be keen on this when taping at other venues...

[Gutbucket]

I'm always happy when then I'm setup and recording before they run a few calibration sweeps and/or pink-noise bursts through the PA.

Have never actually tried to use that in processing the recordings, but there is potential there for EQ and other venue-specific corrections.

[commongrounder]

Hmmm. I agree with Morst. It is highly unlikely a concert venue using PA speaker stacks would use a 70/100v distribution system. This type of system is typically used in zoned areas such as stores, malls, conference rooms, churches, offices, etc. Think arrays of ceiling speakers, not point source stacks. Each speaker has a matching transformer. The impedance checking function is a clever way to monitor line conditions, since it is possible to have a wire short somewhere in a zone and still have some output from the speakers on that zone.
When I read the OP’s  post, the first thing that came to mind was high frequency switching noise from the now common LED lighting systems. The LED PWM driver circuits can generate a considerable level of ultrasonic tones. This is also true of the switching power supplies that are everywhere now. There is a fair amount of discussion about this issue in other recording forums I participate in.

[EmRR]

I didn’t know about the distributed system tone.

In the studio I frequently have an analyzer on, looking at various signals, and it’s wild how much of that kind of thing exists out of hearing range, sometimes in hearing range but not obvious because it’s so low. Sometimes it’s a power supply related proximity effect that changes with location, sometimes it appears to be unknown temporary interference. Sometimes it’s fixed frequency and amplitude, sometimes both change.

[Ronmac]

The only thing that we know for sure is the frequency and amplitude of the tone.

I mentioned the possibility it could be a distributed speaker system pilot tone because I thought it the most likely source. It is not unusual that venues have a distributed speaker system, aside from the main FOH array. Think backstage/green room comms, house music, box office, foyer, washrooms, etc.

[morst]

Kinda off topic but while we are discussing random noise...


Sometimes I get a job at one of the piers in SF, and we have wireless radio frequency mics (usually Shure or Sennheiser systems) so we have to watch out for the giant cruise ships coming by with their 100+ channels of wireless!!!
This is why we have backup frequencies, but it's still a bit of a panic when you "take hits" on your channel.
 

[Gutbucket]

Even more OT, a bunch of the cruise ships departing from here play the Love Boat theme over their big signaling horns, yet not a single one is actually in tune.  While somewhat endearing upon first listen, it quickly becomes annoying for those living nearby.