- Written by John E. Johnson, Jr.
- Published on 10 October 2012
In a conventional DAC, a Pulse Code Modulated (PCM) bit stream, e.g., 16 bit, 44.1 kHz sampling, is decoded into an analog output signal. The bitstream consists of 1's and 0's, in groups of 16, representing a 16 bit word that is a code for a voltage level in the music signal with respect to 0dBFS. In a typical DAC, the output at 0 dBFS is usually about 5 volts. The bits, which are pits and "land" are all of equal duration as the laser scans them while the disc is spinning. For a 44.1 kHz signal, 22 kHz has only two samples, which would otherwise be translated to an analog square wave, which, if dissected, would consist of an infinite number of harmonics. To eliminate the harmonics, and pass just the 22 kHz analog signal, there is a filter at the output that is very steep, and which eliminates all signal above 22.05 kHz.
Sometime around the beginning of the 21st century, engineering articles began appearing that discussed ways of converting PCM to Pulse Width Modulation, or PWM. With this method, the signal remains in digital format all the way to the output. It never actually is converted to an analog signal, but rather, a Pulse Width Modulated signal that resembles the musical wave form.
The PCM-to-PWM conversion approach to digital music reproduction has matured rapidly, and products have begun to appear on the market that use this technique. The NAD M51 is one of those products. It's called a Direct Digital DAC due to its digital signal from input to output.
A Pulse Width Modulated signal is produced by a series of pulses, all of which are at the full rail voltage, but they are of different duration. The M51 takes the incoming PCM signal, and using a clock running at 108 MHz, resamples the PCM at 844,000 samples per second, so each pulse ("period") is 1/844,000th of a second in length, but the voltage is on for varying percentages of that period, depending on the voltage level of the signal, with respect to 0 dBFS. This is controlled by two switches, one for the + and one for the -.
At 22 kHz, instead of only the 2 samples that you would have with a conventional 16/44/1 bitstream, you now have 844,000/44,100 = 38 samples. These are fed to a high precision op-amp. As with a PCM signal that is converted to an analog signal, a PWM output uses a filter to remove any harmonics, and the filter consists of a capacitor and inductor.
Here is a spectrum of a 20 kHz sine wave and the PWM pulses that would represent it at the output: The pulses are indicated by the vertical blue bars, and for the positive portion of the waveform, the + switch is on for a longer period of time than the - switch, and vice versa. The spectrum is based on having a 1.33 μsec delay between the pulse and the voltage at the output, which is a result of the values of the output filter (inductor and capacitor).
The values of the output inductor and capacitor are critical. For example, if the values changed the delay to 1.8 μsec delay, the output waveform would appear as in the spectrum shown below. Note the large amount of ripple.
Besides having 38 samples at 22 kHz, the PWM is 35 bits, so the M51 can be used as a preamp with its digital volume control. With a conventional PCM signal, using a digital volume control truncates the LSB (Least Significant Bits), so music detail is reduced at low volume. With the 35 bit PWM signal in the M51, even with a 24 bit PCM incoming signal, the volume would have to be reduced by 66 dB before truncation of the original music signal would begin to occur.
The front of the M51 is very conservative in appearance, as seen in the photo on the previous page. It has an On/Off button, an LED status window, and an Input Selector button.
The remote control is very nice quality, fits nicely in my (large) hands, and the buttons are shaped differently for locating the one you want in the dark.
The rear panel is full of inputs and outputs. From left to right are one pair (each) of XLR (balanced) and RCA (single-ended) outputs, followed by XLR (balanced), RCA (single-ended coax), Toslink Optical, USB-1, HDMI-1, and HDMI-2 digital inputs. There is also an HDMI video out (pass-through), an upgrade socket, an RS-232 jack, and trigger jacks. The AC connector is grounded (three-prongs).