Audio Alchemy Digital Decoding Engine v1.0 Measurements

Sidebar 2: Measurements

The Digital Decoding Engine performed quite well on the bench. It didn't measure as well as some more expensive units, but nothing in the measurements would indicate its budget heritage.

Driving the DDE with data representing a positive-going impulse revealed it to be non-inverting with the front-panel switch in the 0° position, and inverting in the 180° position. No DC was measured at the output jacks, but there was a very low level (2mV p–p) of high-frequency noise (350kHz) always present at the output. Output impedance was 220 ohms across the band, not a particularly low value, but low enough to drive most passive control units (provided the interconnect is of low capacitance).

The DDE's output level when decoding a full-scale, 1kHz sinewave was 2.46V, 1.8dB higher than the industry standard 2V output level. Frequency response, shown in fig.1, was flat, but with some passband ripple evident above 2kHz, a result of the 7323 Bitstream chip's linear-phase digital filter. De-emphasis error was negligible, with a very slight (0.1dB) positive error at 16kHz. This is well within the specified error of ±0.25dB. Channel separation (fig.2) was fairly good, L–R measuring an excellent 98dB at 125Hz, but decreasing to 80dB at 16kHz. The R–L separation varied less with frequency, measuring 88dB at 125Hz and decreasing to 83dB at 16kHz. All these separation figures, however, are within Audio Alchemy's published specs.


Fig.1 Audio Alchemy Digital Decoding Engine v1.0, frequency response at –12dBFS (right channel dashed, 0.5dB/vertical div.).


Fig.2 Audio Alchemy Digital Decoding Engine v1.0, channel separation (10dB/vertical div.).

Looking next at the spectral content of the DDE when decoding a low-level (–90.31dB) 1kHz tone in fig.3, we can see a moderately high level of noise, and some second- and third-harmonic content. The amount of 60Hz noise was highly dependent on the grounding arrangement: this plot was the best obtained.


Fig.3 Audio Alchemy Digital Decoding Engine v1.0, dithered 1kHz tone at –90dBFS with noise and spuriae (1/3-octave analysis, 10dB/vertical div.).

The linearity plots (the left channel, which was very slightly worse than the right, is shown in fig.4) reveal more error—slightly worse than claimed in the specs—than I'm accustomed to seeing from a Bitstream decoder. Although these plots aren't poor, other Bitstream decoders I've measured have offered lower linearity error below –80dB. However, this performance is still better than most inexpensive multi-bit decoders. Many digital processor manufacturers would be happy to trim their converters to this level of linearity performance.


Fig.4 Audio Alchemy Digital Decoding Engine v1.0, left-channel departure from linearity (2dB/vertical div.).

A –90.31dB, 1kHz dithered sinewave, captured by MLSSA, is shown in fig.5. The 1kHz signal is just apparent, but is overlaid with a fairly high level of audioband noise. Transformed to the frequency domain, the associated spectrum is shown in fig.6, which reveals a degree of second harmonic distortion present. Fig.7 is the DDE's interpretation of a 1kHz, full-scale squarewave. The unclipped overshoot and Gibb's Phenomenon ringing are typical of a Philips digital filter.


Fig.5 Audio Alchemy Digital Decoding Engine v1.0, waveform of dithered kHz tone at –90.31dBFS.


Fig.6 Audio Alchemy Digital Decoding Engine v1.0, FFT-derived spectrum of dithered 1kHz tone at –90.31dBFS.


Fig.7 Audio Alchemy Digital Decoding Engine v1.0, 1kHz squarewave at 0dBFS.

The DDE showed a low level of intermodulation products when decoding a full-scale combination of 19kHz and 20kHz tones, with no evidence of any 1kHz product and only very slight "spurs" at 18 and 20kHz (fig.8). The cursor shows the level of the 24.1kHz (44.1kHz sampling –20kHz signal) product, which is reasonably well suppressed. (The Japanese filters from NPC, Yamaha, etc., offer greater suppression of this product, but clip the peak ringing on full-level squarewaves.)—Robert Harley


Fig.8 Audio Alchemy Digital Decoding Engine v1.0, HF intermodulation spectrum, 0Hz–30kHz, 19+20kHz at 0dBFS (linear frequency scale).

Audio Alchemy, Inc.
Company no longer in existence (2014)