basic incarnation, it had two line-level, balanced inputs. One popular upgrade was the addition of one or two microphone preamps, which were installed on removable cards into slots in the machine. These allowed stereo recording directly into the Model 700, bypassing a mixing console. Since the recorder had a significantly lower noise floor than most mixers of the same era, this method made the best use of the system's available dynamic range. Another, much more rare accessory was the Model 700D Disc Mastering Delay. This was a device used for mastering vinyl records, and attached to a proprietary 25-pin digital output on the back of the Model 700 recorder. Because of the nature of vinyl records (which rotate at a constant angular velocity but at a changing linear velocity with respect to the needle as it moves inwards), it is necessary to send the audio signal to the computer controlling the movement (pitch) of the cutting lathe. In this way, during quiet passages the pitch is decreased thus enabling more grooves per inch. The computer needs an audio signal that is exactly one rotation ahead of the actual audio fed to the cutting lathe, so that it can move the cutting head rapidly forward before a loud passage is grooved. The disc mastering delay achieved this just like the analog mastering tape decks that used an extra playback head for this purpose. The Model 700 was developed and sold by the dbx corporation of Newton, Massachusetts, better known for their system of noise reduction for analog tape. Technical specifications Dynamic range: 110 dB typical with A-weighted noise 20 Hz–20 kHz; >105 dB unweighted Frequency response: 20 Hz–20 kHz, sine or pink noise, 100mV, reference record position Total harmonic distortion: less than 0.05%, 1V input, 1 kHz Wow and flutter: less than 0.01% unweighted; 0.006% wrms Anti-alias filtering: −3 dB at 37 kHz Sampling rate: 644 kHz Bit rate: 644 kbit/s Mic preamp: adds less than 1 dB noise, 100 to 1k-ohm impedance Max in/out levels: +24 dBm Theory of operation The Model 700 converted analog audio into digital data using a type of delta-sigma modulation, called "Companded Predictive Delta Modulation," or "CPDM" (both trademarked). In a traditional, single-integrated delta-sigma ADC, the voltage of an input signal is compared to the output of an integrator. If the input signal is higher than the integrator's output, a 1 is recorded, and the