M-Audio Torq 2.0 Руководство по эксплуатации онлайн [165/171] 761258

Appendix B: Torq Vinyl Control System

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Torq 2.0 User Guide

Conversely, if you speed up a record, the music will rise in pitch. Torq uses this phenomenon to calculate play-

back speed by listening to the pitch of the pilot tone. If the pitch drops, that means that the record has been

slowed down-Torq then slows down the playback of the audio file by the same amount. If the pitch goes up,

that means the record is spinning faster and Torq increases the playback rate of the music. If Torq hears the

pilot tone at its original pitch, it means the record is playing at its normal speed (33 1/3 RPM), thus Torq plays

the song at its original speed.

By listening to the pitch of the pilot tone, Torq can determine speed. But how does it determine direction? The

answer is stereo phase-shift. Records are stereo devices and can encode two channels of audio (left and right).

The pilot tone is printed onto the record with the left and right channels 90-degrees out-of-phase. When the

record plays forward, the right channel will be 90-degrees ahead of the left channel. When the record plays

backwards, the right channel will be 90-degrees behind the left channel. Torq evaluates this phase relation-

ship to determine direction of playback.

The pilot tone mechanism explained above allows the computer to generate a Velocity value. The velocity is

both speed and direction expressed as one number. When the Velocity is 1.000, the record is spinning forward

at normal speed. When the Velocity is -1.000, the record is spinning backwards at normal speed. If the Veloc-

ity is 0.500, the record is spinning forward but at half its normal speed. If the Velocity is 0.000, the record is

stopped. Torq displays the Velocity value for each Deck in the External Control Preferences.

The last piece of information needed to properly emulate vinyl control is Position. While you can do scratch-

ing just using the Velocity value, you won't be able to perform needle drops or prevent “sticker drift” without

using the Position information. Position is one of the most difficult pieces of the vinyl control puzzle and is

also the mechanism that differs most from one vinyl control system to the next. Torq employs a system of po-

sition stamps in order to determine playback location on a record.

The position stamps are digital numbers encoded as analog audio signals which are placed on the record at

regular intervals. In the case of the Torq Control Vinyl, there are about 155 stamps for each revolution of the

record. As the stylus passes over a position stamp, Torq can decode it to determine location. Furthermore, the

decoding can occur if the record is playing forwards or backwards

The Position value is not only determined by the position stamps. Instead, Torq uses the position stamps and

Velocity together to determine Position. The reason for this is that the position stamps are “coarse.” That is,

they only occur at periodic intervals across the record instead of being continuous, and there is a certain

amount of “empty space” between each stamp. How can Torq determine position if the needle is between two

position stamps? The answer is interpolation. Torq can track Velocity between two stamps (thanks to the con-

tinuous pilot tone). Therefore, if Torq reads a little bit of movement after passing over a position stamp, the

actual Position will be a value between that position stamp and the next. This final value is displayed as Po-

sition on Torq's External Control Preferences.

So there you have it: a continuous phase-shifted pilot tone for determining Velocity and thousands of tiny po-

sition stamps for determining Position. In theory, this all sounds great, doesn't it? In practice, things are a little

more difficult. There are a number of factors that fight this system's reliability including stylus quality, clean-

liness, audio isolation, ground hums, and wiring.