Tuesday, July 12, 2011

Does Audio Cable Length Matter

The length of your cables does matter. Not because the signal gets delayed if it travels through a longer cable or because it might get phase-shifted compared to the same signal traveling through a much shorter cable. The effect of the wire and cable lengths in the recording studio are so insignificant to the delay of the signal that we can just ignore them. (it takes about 0.00089 seconds or almost 1ms (millisecond) for sound to travel 1 ft in air at sea level. In order to get your signal delayed by 1ms just by adding length to your cables, it would take you about 186 MILES of cable)
The length of the cable does matter though especially if we are talking about a guitar unbalanced cable. The impedance of the guitar is so high that the capacitance of the cable would definitely affect the high end of your signal. See Gene Dellasala's detailed article on RLC losses in a cable. Also Unbalanced connections are susceptible to magnetic and Radio Frequency interference so the longer the cable the better the chances are you'll end up with unwanted hum or radio or who knows what in your guitar signal.
If we are talking about balanced microphone cable the length matters for the same reasons as the guitar cable although balanced cables tend to reject magnetic interferences pretty well. The problem with the microphone signal is that the voltage of it is so low (in the micro Volt range - 1 micro Volt = 0.000001 Volts) that after amplifying it 40-50 dB or so it is very important that the cables pick up absolutely nothing on the way to the mic-pre input. Also I personally had the weird experience with the Sony "Solid Tube" Mic (more than once on 2 different mics) where I had a 3 ft "Mogami" mic cable coming out of the power supply straight in to my studio's mic panel on he wall, and I was picking up some radio station. First I lifted the ground on the power supply but it got worse. Then I changed the cable with a 15ft Mogami and the radio disappeared. Now I always use the longer cables on the "Solid Tube" until one day I had that same radio... I changed the cable back to the 3-footer and the radio was gone. Maybe just a coincident or some capacitance match or who knows what... My point is CABLE LENGTH MATTERS.
If we are talking about speaker cable, a lot of people have the miss-conception it's OK to run Speaker Cables as long as you want. It is both true and not. Speaker level has the highest voltage and the lowest impedance out of all signal levels used in a recording studio. In that sense it is safest to run the longest runs. On the other hand, because of it's super low impedance the resistance of the wire makes a big difference on the power at the other end. Depending on the speaker cable a 50 ft run, depending on the gage, may have a total of 2 to 4 Ohms of resistance. If your speaker's impedance is 4 ohms and your amplifier is rated to drive 100 watts over 4 ohms (assuming) the wire resistance is 0 ohms, you'll get your full 100 Watts. What happens when you put your speakers 50 ft away from the amp... the resistance of the cable gets added to the 4 ohms of impedance of the speaker for a total of 8 ohms. Now the same 100 Watt at 4 ohms amp has 8 ohms of load instead of 4 ohm giving us only 50 Watts. Power(Watts) = E squared divided by the Resistance (impedance) where E=Voltage which is constant in the above example. Note the use of resistance vs impedance and vice-versa. We use resistance when we are referring to DC circuits, and Impedance when talking about AC circuit - all audio signals are AC (Alternating Current)
 See also the following blogs on this topic:

Saturday, July 2, 2011

Phase Vs. Polarity

Phase has to deal with timing and waveforms developing over time.
Polarity on the other hand has nothing to do with timing. It is purely a matter of direction of flow of electrical current.
You've probably heard the term "Out of Phase" or "180ยบ Out of Phase" 99% of the time people use this term when they mean "Reversed Polarity" Almost every mic preamp or a console out there has a "Phase flip" switch, which is misleading. All that switch does, is reverse the polarity of the signal. Since it clearly has nothing to do with the timing of the signal it should not be associated with "Phase". 

Phase problems or issues occur every time we have two or more instances of one source of sound. For example…
A guitar miked with more than one microphone, or a direct signal from a bass and a split of that same bass going through a bass amp which we've miked, or a close-miked drum-set and a room mic picking up that same drim-set. In all of the above examples we have one source of sound and more than one physical location where we capture that sound. Considering the speed of sound in air (1125 ft/s) it makes sense that one of our instances of that source would run slightly behind the other - (timing differences equals phase differences). The effect of the wire and cable lengths in the recording studio are so insignificant to the delay of the signal that we can just ignore them. (it takes about 0.00089 seconds or almost 1ms (millisecond) for sound to travel 1 ft in air at sea level. In order to get your signal delayed by 1ms just by adding length to your cables it would take you about 186 MILES of cable)
So you have your two mics on your kick drum for example - (one inside and one outside of the drum) about 1 ft away from each other. The outside mic will pick up the sound of the beater hitting the head about 1ms later than the mic a foot closer to the beater. So considering the speed of sound again, frequencies around 1125 Hz may get a boost while frequencies around 562.5 Hz may get lost. If we reverse the polarity of one of the two mics in theory the frequencies around 562.5 should get a boost and the range around 1125 Hz should disappear. Yes and NO. If we work with pure sine waves that would be the case - Before we reverse the polarity of the mic 1025Hz sine wave would fall exactly 360 degrees shifted over the distance of 1 ft and double itself in amplitude, while 562.5Hz sine wave would shift exactly 180 degrees over 1ft, so it would cancele itself completely. But since we are dealing with a complex waves and not just a pure sine wave, plus the fact that the two mics are probably different models and they pick up the sound of the drum at different locations of the drum itself, the two signals would have different frequency content to begin with. So after we combine them together we get a third frequency content of the sound of the drum. By flipping the polarity of one of the mics we drastically change the combined frequency content but that doesn't mean we completely lost our 1kHz range. Depending on the drum the mics the positions and also the rest of the drum mics like over heads and rooms you may end up reversing the polarity of one or two mics so you can get the desired sound... There is no right or wrong way... Just experiment and trust your ears. Rule of thumb is If you have two mics that are close to each other - 6 inches or less and you would like to mic some kind of a low frequency source - bass amp kick drum, guitar amp etc. make sure the polarity on of both mics is the same. After summing or mixing two microphones that have timing/ phase differences in the sound they are picking up, we create what's called a "Comb Filter". By reversing the polarity of one of the microphones all we do is shift the frequencies that get boosted and the ones that get cancelled out. In either case we end up with a severe EQ curve. At this point It is just a matter of preference and overall desired sound.

Also see article by "Musician's Friend" on the same topic.