What It All Means
Where we sort out and explain carefully a set of technical terms: this month, recording gear
Manufacturers spend months perfecting accurate spec sheets, but how much of them do we understand? Each month WHAT IT ALL MEANS will take familiar technical phrases from one section of the gear market and attempt to make them make sense.
BIAS: to get the full range of frequencies efficiently on to the tape, the upper range needs some assistance... a leg up. A high frequency bias (above 50kHz) is added along with the signal. When judged correctly (thankfully the maker's job, not yours) the bias will help reduce distortion and lower the background noise. The spec may quote the bias frequency, but it shouldn't affect the day to day performance. (In other words, don't worry.)
CROSSTALK: commonly measured at a tone of 1 kHz, it shows the level at which the signal on one track can be heard on the next. The closer you squeeze the tracks, the more crosstalk you'll get. Between 50 and 55dB are good averages, but on a small machine like the Fostex X-15 you'd expect 40dB, and on a professional reel to reel, you'd want 65dB or above.
DI: direct injection. Taking the lead from your guitar and plugging it straight into the mixer would be direct injection. Connecting it to an amp, then miking the resulting noise, wouldn't
DOLBY: a system of noise reduction. Dolby proper splits the signal into various frequency bands, and during recording dramatically boosts the high frequencies. On playback they are returned to their original strength and any hiss that went with them is forced down into (hopefully) inaudible levels. DBX treats the signal as a whole, without splitting it into bands, and is preferred by some recordists as having fewer side effects on the final sound. Dolby A was the first professional system and is used in studios; B and C are alternatives developed for hi-fi and four track machines.
EIA: Electronic Industries Association, a test standard.
EQUIVALENT NOISE INPUT (EIN): it's easiest to measure unwanted noise at the output of a deck, even though most of it is generated at the input stages, especially the microphone circuits. Taking your output noise figures and dividing them by the amount of amplification between input and output gives you the equivalent input noise. Expect between -100 and -120dB.
FREQUENCY RESPONSE: how much of the audible range of sound the equipment can cope with, and how smoothly it deals with it. Quoted in Hertz (Hz) but should really be qualified to be of the most use. "20Hz to 20kHz"... sure, but what's to say that at 20kHz the volume hasn't dropped by a factor of 12. Best specs should be followed by a figure like -3dB or +1dB showing by how much the level varies at the most impressive frequency. The smaller the figure (ie the closer to zero) the better.
HUM: not surprisingly, an unwanted, low-pitched drone, at frequencies which are harmonics of the mains supply (50Hz in the UK), and emanate from it
INPUT SENSITIVITY: the minimum input signal required to produce an output signal demonstrating the desired signal to noise ratio.
IHF: Institute of High Fidelity, a test standard.
IPS: inches per second, a measure of tape speed. The average hi-fi cassette deck runs at 1⅞ths ips, some four-track versions double this to 3¾ to improve response. Philips hold the patents on cassette, speeds and certain other larger manufacturers are wary of running at 3¾ for fear of infringing them.
NAB: National Association of Broadcasts, a test standard.
PARAMETRIC EQ: two controls, one affects the level, the other homes in on the band of frequencies you want cut or boosted. Like a tone control, but you get to choose which 'part' of the tone is affected... bass between 50Hz and 200Hz, mid-range vocals around 5kHz, to "fizz" at 10kHz, etc.
ROLL OFF: how the frequencies fall off in level. Eg a bass roll off means the bass will decrease in strength pass this point
SIGNAL TO NOISE: the size of your inputted signal compared to the size of the unwanted noise. It's shown as a ratio, expressed in decibels. (Can vary from 60dB without noise reduction to 85dB with it; 65dB is acceptable, the higher the better).
TOTAL HARMONIC DISTORTION: an unwanted signal or signals harmonically related to the original input not just random noise. You can get away with the lower harmonics (2nd, 3rd, 4th) but the higher versions (7th, 8th, 9th) are unpleasant. However, detection equipment cannot easily separate the different harmonics out so you only get their sum. Ideally, specs should tell you how the THD performs throughout the frequency range - it can vary. Fabulously brilliant figures at one specific frequency could be misleading. Disaster may lurk either side of that number.
TRANSIENT: a sudden and short lived burst of volume (generally) which the ear can detect but is too rapid for meters to display. For example, if you twang a guitar string hard the first split second will be dead loud (so the brain knows there's a noise out there with a percussive punch). It then drops to the generally perceived level which lets you work out pitch information, and so on. The needle of your meter displays the second bit but cannot react speedily enough to illustrate the first. However, transients do exist and can create distortion on the tape.
WEIGHTING: a word you'll see attached to many parts of the spec. It means the measurements have been artificially adjusted to take account of conditions which would exist when the machine is in normal use, but aren't around on the test rig. In other words, fiddled with to give real life values.
WOW AND FLUTTER: both are unwanted variations in pitch, invariably introduced by transport machinery. Wow is a slow, steady change (say on each revolution of the cassette) and is most noticeable on low frequencies. Flutter is a faster, and often less regular variation, hitting all frequencies.
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