Sidebar 2: Measurements
On the filter 1 setting (fig.1), the response was essentially flat, measuring –0.2dB at 20kHz. On 2, the response fell away beyond 3kHz to –2dB, 20kHz. With filter 3, the output shelved just a little, this being the designer's preferred characteristic. On 4 (1 and 3 depressed), the bass rolled off early by 2dB into our low-impedance chart-recorder loading, while the high treble was slightly depressed—anti-fuzz, perhaps? On 5, the treble rolloff accelerated, while 6—all filter buttons depressed—gave a maximum level of cut at both extremes.
Fig.1 Cambridge Audio CD1, frequency response with (from top to bottom), FIlters 1–6 (1dB vertical div.).
Channel balance was excellent at 0.07dB, 1kHz; channel separation was also excellent—worst case 105dB at 20kHz!—as was the interchannel phase match.
The distortion results were fine at high levels (fig.2 shows the upband products for a full-level 20kHz tone), and were even better still at –60dB modulation (fig.3), as well as at –80dBFS, where the distortion of a 1kHz tone measured –27dB. This correlated well with the minimal level error at –90dBFS, which averaged less than 0.4dB, confirming true 16-bit resolution. The –90dB sinewave form (fig.4) was a bit noisy, but a reasonable shape may be seen underneath the noise.
Fig.2 Cambridge Audio CD1, spectrum of 20kHz at 0dBFS, 0Hz–100kHz (12dB/vertical div.).
Fig.3 Cambridge Audio CD1, spectrum of 1kHz at –60dBFS, 0Hz–10kHz (12dB/vertical div.).
Fig.4 Cambridge Audio CD1, waveform of dithered 1kHz sinewave at –90dBFS (4ms time window).
The output is higher than usual at 4.3V, sourced from a low 110 ohms, which rose to a maximum of 4k ohms from the variable output.
The rejection of ultrasonic spuriae was unexceptional at a typical 53dB, but no problems were encountered with respect to error correction, de-emphasis, or mechanical noise. Electrical signal/noise ratios were excellent at 113dB or so. The impulse response (fig.5) showed a non-inverting, linear phase result.—Martin Colloms
Fig.5 Cambridge Audio CD1, impulse response, 44.1kHz sample rate.
