Sidebar 3: Measurements
I performed a full set of measurements on the Bricasti M21 using my Audio Precision SYS2722 system. The coaxial S/PDIF, optical TosLink, and AES3 inputs all accepted data sampled at rates up to 192kHz. Apple's AudioMIDI utility revealed that the USB port accepted 32-bit integer data sampled at all rates from 44.1kHz to 768kHz. The USB Prober app identified the M21 as "MDx" from "Bricasti" and indicated that the USB port operated in the optimal isochronous asynchronous mode. When I connected the Bricasti's Ethernet port to my network, Roon recognized the processor as "Bricasti M12 Source Controller." I used optical, USB, and network PCM data for the tests. For logistical reasons, it wasn't possible to perform any testing with DSD data.
I used the remote control to set the Bricasti's output level to "0dB" because the manual says that with this setting, the M21 is placed in "hard wire bypass mode," allowing the processor to be used "as a direct pure DAC." The output level with a 1kHz signal at 0dBFS was 4V balanced and 2V single-ended, sourced from impedances of 58 ohms and 29 ohms, respectively; both values were consistent from 20Hz to 20kHz. The M21 preserved absolute polarity from the balanced and unbalanced outputs with Phase set to noninverting.
Fig.1 Bricasti M21, Sigma-Delta DAC, impulse response with the Linear1 filter (left) and Minimum1 filter (right) (one sample at 0dBFS, 44.1kHz data, 5ms time window).
There are four different types of oversampling reconstruction filter, labeled on the front-panel display as Linear0, Linear1, Minimum0, and Minimum1. The two Linear filters had identical impulse responses, shown to the left of fig.1; this impulse response is typical of a long linear-phase filter, with equal amounts of ringing before and after the single sample at 0dBFS that was used to generate this graph. The two Minimum filters also had identical impulse responses, shown to the right of fig.1; this response is typical of a long minimum-phase filter, with all the ringing following a single sample at 0dBFS. All the filters behaved identically whether I performed this test with the DAC set to Ladder or to Sigma-Delta.
Fig.2 Bricasti M21, Sigma-Delta DAC, Linear1 filter, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan) into 100k ohms with data sampled at 44.1kHz (20dB/vertical div.).
Fig.3 Bricasti M21, Sigma-Delta DAC, Linear1 filter, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan) into 100k ohms with data sampled at 44.1kHz (20dB/vertical div.).
The magenta and red traces in fig.2 show the Sigma-Delta DAC's wideband spectrum with 44.1kHz white noise data at –4dBFS with the Linear1 filter. The response rolls off sharply above the audioband, with full stopband attenuation reached at exactly half the sample rate. The reconstruction filter is therefore an apodizing type, and the image at 25kHz of a full-scale 19.1kHz tone (cyan, blue traces) is completely suppressed. While the third harmonic of the 19.1kHz tone can be seen, it lies at just –90dB (0.003%). The Minimum1 filter behaved identically. When I repeated this spectral analysis with the Ladder DAC (fig.3), the behavior of the reconstruction filters was identical to that of the Sigma-Delta DAC, but now many low-level spurious tones were present in the noisefloor, and the second and third harmonics of the 19.1kHz tone were higher in level at, respectively, –73dB (0.02%) and –66dB (0.05%).
Fig.4 Bricasti M21, Sigma-Delta DAC frequency response at –12dBFS into 100k ohms with data sampled at: 44.1kHz (left channel green, right gray), 96kHz (left cyan, right magenta), and 192kHz (left blue, right red) (1dB/vertical div.).
Fig.5 Bricasti M21, balanced, spectrum of 1kHz sinewave, DC–1kHz, at 0dBFS (left channel blue, right red; linear frequency scale).
The Bricasti M21's frequency response with 44.1kHz, 96kHz, and 192kHz data was flat in the audioband (fig.4) and followed the same basic shape at all three sample rates, with a very sharp rolloff just below half of each rate. The frequency response was identical from both output types with both types of DAC and all four reconstruction filters. Channel separation was superb, at >120dB in both directions below 3kHz and still 116dB at the top of the audioband. The low-frequency noisefloor was very low in level and was free from power supply–related spuriae with both types of DAC (fig.5).
Fig.6 Bricasti M21, Sigma-Delta DAC. left channel, 1kHz output level vs 24-bit data level in dBFS (blue, 10dB/vertical div.); linearity error (red, 1dB/small vertical div.).
Fig.7 Bricasti M21, Ladder DAC. left channel, 1kHz output level vs 24-bit data level in dBFS (blue, 10dB/vertical div.); linearity error (red, 1dB/small vertical div.).
The red traces in figs.6 & 7 respectively plot the error in the balanced output level as a 24-bit, 1kHz digital tone steps down from 0dBFS to –140dBFS in Sigma-Delta DAC and Ladder DAC modes. The amplitude error is extremely to below –130dBFS in Sigma-Delta mode, which implies very high resolution. In Ladder DAC mode the error starts to increase below –110dBFS and rises significantly below –118dBFS.
Fig.8 Bricasti M21, Sigma-Delta DAC, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with 16-bit data (left channel green, right gray) and 24-bit data (left blue, right red) (20dB/vertical div.).
Fig.9 Bricasti M21, Ladder DAC, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with 16-bit data (left channel green, right gray) and 24-bit data (left blue, right red) (20dB/vertical div.).
Fig.8 shows the M21's balanced output spectra with 16- and 24-bit dithered data representing a 1kHz tone at –90dBFS with the Sigma-Delta DAC. The increase in bit depth lowered the noisefloor by 20dB, which suggests a measured resolution of between 19 and 20 bits. While the level of the random 24-bit noisefloor was just as low with the Ladder DAC (fig.9), many harmonic distortion products were now present, suggesting that the data were being truncated.
Fig.10 Bricasti M21, waveform of undithered 16-bit, 1kHz sinewave at –90.31dBFS (left channel blue, right red)..
Fig.11 Bricasti M21, waveform of undithered 24-bit, 1kHz sinewave at –90.31dBFS (left channel blue, right red).
With both types of DAC and undithered data representing a tone at exactly –90.31dBFS, which consists of data at –1LSB, digital zero, and +1LSB, the waveform was symmetrical, and the three DC voltage levels described by the data were readily resolved (fig.10). With undithered 24-bit data the result was a noise-free pure sinewave (fig.11).
Fig.12 Bricasti M21, spectrum of 50Hz sinewave, 24-bit data, at 0dBFS, DC–1kHz, into 100k ohms (left channel blue, right red; linear frequency scale).
Fig.13 Bricasti M21, Sigma-Delta DAC, HF intermodulation spectrum (DC–30kHz), 19+20kHz, 24-bit data, at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).
The Bricasti M21 Sigma-Delta DAC's distortion signature with data representing a 50Hz signal at 0dBFS (fig.12) primarily comprised the second harmonic, but this was very low in level, lying at –106dB (0.0005%). Intermodulation distortion with 24-bit data representing an equal mix of 19 and 20kHz tones, each at –6dBFS, was also extremely low in level with the Sigma-Delta DAC, even into 600 ohms (fig.13). The difference product at 1kHz lay at –100dB (0.001%). This product rose to –76dB (0.015%) with the Ladder DAC and was joined by higher-level, higher-order products and many low-level spurious tones.
Fig.14 Bricasti M21, Sigma-Delta DAC, 16-bit optical data, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
Fig.15 Bricasti M21, Ladder DAC, 16-bit optical data, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
The Bricasti M21's Sigma-Delta DAC was immune to data-related jitter with all its inputs. Fig.14 shows the spectrum of the M21's output when it was fed 16-bit optical J-Test data. The odd-order harmonics of the undithered low-frequency, LSB-level squarewave all lie at the correct levels, and the noisefloor between those harmonics is appropriately low in level. However, the spectral spike that represents the high-level tone at one-quarter the sample rate is broadened at its base due to low-level, low-frequency random jitter. The Ladder DAC did less well with this test, with the odd-order harmonics of the undithered low-frequency, LSB-level squarewave too high in level (fig.15).
The Bricasti M21's measured performance in Sigma-Delta DAC mode was superb, with high measured resolution, very low distortion, and excellent rejection of data-related jitter. Relative to Sigma-Delta mode, the Ladder DAC mode offered compromised measurements.—John Atkinson
