R/E/P Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: 1 [2]  All   Go Down

Author Topic: cell phones in the studio.  (Read 6489 times)

stickman

  • Newbie
  • *
  • Offline Offline
  • Posts: 36
Re: cell phones in the studio.
« Reply #15 on: May 15, 2004, 11:42:35 pm »

 Laughing definitely wasn't the talkback. also, i can hear the other person's voice (on the other end of the line). the whole conversation was there from that sound before the phone rings - you know that pulsy sound you hear in electronics just before it rings - to the end of the conversation.

oh and it wasnt a walkie talkie analogue brick in my hand... honestly, im not making this up!
Logged

josh

  • Full Member
  • ***
  • Offline Offline
  • Posts: 191
Re: cell phones in the studio.
« Reply #16 on: May 17, 2004, 11:24:35 am »

stickman wrote on Sun, 16 May 2004 04:42


oh and it wasnt a walkie talkie analogue brick in my hand... honestly, im not making this up!


Hm.  Maybe you have a dual-mode phone that was in analog mode when the call was made for some reason (insufficient signal from digital network, etc.?)...

Otherwise, that's a big "dunno"...  digital cell phone encryption should make this pretty much impossible.  Even a scanner tuned exactly to the correct frequency will only give you hash/noise from a call made with a digital cell phone.

I think we can rule out RF interference or pickup if in fact the call was a digital cell phone call, simply because this would not be the observed effect.  So either it's an analog call and it could have been (very unlikely, but possible) RF interference, or it's some other phenomenon like magnetic coupling from the speaker magnet to a magnetic pickup nearby...  tape head, guitar pickup, etc., but you'd think the proximity would have to be VERY close and you'd have to turn up the vol on the cell phone speaker VERY high.

imaginarymusic

  • Newbie
  • *
  • Offline Offline
  • Posts: 3
Re: cell phones in the studio.
« Reply #17 on: May 17, 2004, 12:12:20 pm »

i think that the reason the noise happens most when the phone is ringing is because the cell tower and phone are "looking" for each other and until a stable connection is established there is a stronger signal.  
Logged

stickman

  • Newbie
  • *
  • Offline Offline
  • Posts: 36
Re: cell phones in the studio.
« Reply #18 on: May 17, 2004, 10:56:16 pm »

well a couple things just entered my mind which i remember now (it was quite a while ago when this happened).

my initial thoughts were that perhaps there was an equipment malfunction. the console had just been re-wired (not by a professional) in the studio only a couple weeks before.

and, at the time, the phone i had was in a bad state. the aeriel was half hanging out and was sort of only gaffer taped on  Rolling Eyes

also, i remember (and i really dont know why i remember this) that i taking the call with my head right next to a speaker while i was trying to fix something at the console.

now josh, its quite obvious i know nothing about cell technology but could any of these things have accounted for the mishap?

and thanks for all the great info so far...
Logged

josh

  • Full Member
  • ***
  • Offline Offline
  • Posts: 191
Re: cell phones in the studio.
« Reply #19 on: May 18, 2004, 11:12:32 am »

stickman wrote on Tue, 18 May 2004 03:56


now josh, its quite obvious i know nothing about cell technology but could any of these things have accounted for the mishap?

and thanks for all the great info so far...


It may by now be quite obvious that I don't know much about it either Wink

Yes, imho, these all could have had an effect.  Let me do my best Fox Mulder:

1.  If the phone had an antenna, it likely had an analog mode...  if the antenna was nearly broken, it could likely have been resorting to analog mode for the call at hand, in which case, radio interference could have caused the anomaly

2.  Regardless of whether the phone was operating in digital mode or analog mode, close coupling of the handset's speaker to the loudspeaker in the room could have caused magnetic interference, which could have coupled back through the amplifier circuit by some means and made it onto the tape.

3.  Of course the presence of a ground loop or unterminated shield or cable could exacerbate the probability of either #1 or #2.  A ground loop will improve the odds of magnetic coupling, and unterminated shields or interconnects (like a mic cable connected to the board but not connected to a mic) will improve the odds of electric field (radio frequency) coupling.


OK ... so in summary, for indie rock recording engineers, here's the scoop on RF/EM interference:

RF or "radio frequency" interference occurs as a result of electric field transmission.  Electric fields are those whose impedance is greater than 277 ohms, thereby transmiting efficiently over air which has a characteristic impedance of 277 ohms.  Accordingly, electric fields are produced by creating a large voltage into a high impedance antenna (high impedance relative to ground).  Electric fields are therefore picked up by high-impedance antennae, which means wiring etc. that has a high impedance to ground (higher than 277 ohms), such as an electric guitar, any "unbalanced" signal, etc.  Bear in mind outputs on amplifiers are typically low impedance and inputs are high impedance, unless there is a matching transformer or some other terminating device in the amplifier.  Effective shielding can prevent this type of interference.  Electric fields can have high intensity over a long distance.  Cell phones, cordless phones, garage door openers, AM/FM radio, pretty much anything using radio, uses electric fields for transmission.  You can think of this as "capacitive" coupling, and it is caused by voltage.

EM or "electromagnetic" interference occurs as a result of magnetic field coupling, which happens as a result of high-current, low-impedance (less than 277 ohms) circuits which have relatively high inductance.  These fields are received by inductive (magnetic) pickups like guitar pickups, dynamic microphone voice coils, tape heads, or ground loops or other "loop antenna" type sources (that is a conductive loop of material that has some meaningful inductance).  Cell phones and other devices can emit electromagnetic fields, but they have high intensity only very close to the source of the field, and are related to current.  If you have a low-power device that operates at a high voltage (relatively), such as a condenser mic, it is not going to create magnetic fields of any significance.  A high-power device (relatively) operating at a low voltage, such as a CPU in a modern PC, will potentially create large magnetic fields.  Low-frequency energy (such as 60 Hz hum) is generally magnetic, not electric field.  Proximity and grounding improvements are the best way to combat this type of interference.  You can think of this as "inductive" coupling, and it is caused by current.

Specifically for cell phones, digital cell phones operate in the 900-2500 MHz range and intentionally radiate electric fields.  The same is true for modern cordless phones.  Digital cell phones use data encryption and do not, obviously, transmit an analog signal, so if it's an intelligible conversation being picked up, or anything audio (other than just flat out noise), then it's not the electric field from a digital cell phone that's being picked up.  A regular cordless phone may not use encryption and may be analog.  They vary.  Other devices that use analog transmission may be things like FRS radios (which operate in the 49MHz range), CB radio (pretty rare these days), garage door openers (again 49MHz), baby monitors, wireless guitar rigs, and of course broadcast TV and radio.  Suffice to say if you have an electric field pickup problem, you are going to hear TV or radio stations which are present almost everywhere in high intensity long before you are going to pick up a relatively weak cell phone, FRS, cordless phone, etc., signal.

For the most part, RF shielding is a misnomer and not really effective in recording studios, because there simply is not a RF interference problem to begin with...  Audio does not occur at those frequencies, and if you have a device which by happenstance can recieve a modulated signal like FM radio then even the most rudimentary shielding will completely fix it.  60Hz hum is not a RF interference problem nor is it one that can be fixed with shielding.  In fact, to minimize RF and EM field coupling, grounding is the best solution, and circuit layout is #2.

The reason balanced interconnects don't "hum" as much as unbalanced interconnects is because by design, they cancel hum.  They pick up just as much hum.  It has nothing to do with the shielding.  If you ground the "N" wire in a balanced circuit, you will hear the hum, regardless of shield, although it will be less than a 2-conductor unbalanced cable simply because the ground impedance is lower.  The reason "shielded" cables seem to hum less than non-shielded cables is simply because the shield itself is a lower-impedance, lower-inductance ground conductor than a wire would be.  This is simply grounding, not shielding.  In fact, improved shielding would be worse for grounding...  how many of you have noted that "braided" shielded cables are quieter than "foil" shield (which is aluminum foil)?  I can tell you difinitively that foil shield is far and away a better shield than braided shield, but it is not a lower impedance ground conductor.

I could write a book on shielding and grounding for audio, maybe I should.  But I'll quit now and look out if anyone has other questions.

j.hall

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3787
Re: cell phones in the studio.
« Reply #20 on: May 18, 2004, 11:23:08 am »

not to open this can of worms, but if your studio has a proper "audio" ground as a portion of the AC power scheme, and building grounding scheme, you need to lift the shield at one end of each cable, to avoid creating ground loops.

if you don't have the slightest clue what i'm talking about, then you probably don't have an "audio ground" and need to connect the shield all the way through.

for those of us who travel around to different rooms with gear of our own, you need to ask if this has been done.

my protable bays, and snakes are are wired up connecting the shield through out.  

another quick check when i arrive is to see if the console (i do this while hooking up my monitors) has a heavy gauge "green" wire landed on a grounding lug.  i'll also check a few other key pieces of gear for this same thing.  that tells me if the gear is on an isolated ground or not......
Logged
Pages: 1 [2]  All   Go Up