Linn Knekt Kivor hard-disk multizone music system Measurements
To assess the Linn Oktal's performance, I ripped a CD-R containing all of my regular 16-bit CD-player test signals. The Tunboks drove the Oktal via a 1m Cat.5 AES/EBU link. I also fed the Oktal 24-bit digital data via one of its S/PDIF data inputs. Unfortunately, the processor would not lock to 96kHz-sampled data, though it did to an 88.2kHz datastream, after a few burps and hiccups.
At 1.942V, the maximum analog output level was inconsequentially below the specified 2V RMS, this sourced from a consistent 211 ohms across the audioband. The Oktal preserved correct absolute polarity. The frequency response (fig.1, bottom pair of traces above 1kHz) was flat within the audioband for both 44.1kHz and 88.2kHz sample rates, with a very slight top-octave rolloff continuing above the audioband at the higher sample rate. However, pre-emphasized data (fig.1, upper traces) were not decoded with the appropriate de-emphasis, presumably because the Tunboks does not preserve the relevant datastream subcode flag. Fortunately, pre-emphasized CDs are extremely rare these days. The Oktal did recognize HDCD data, however. Channel separation (not shown) was excellent below 600Hz, being buried in the noise floor, but deteriorated at 6dB/octave above that frequency to a still-good 80dB at 20kHz, due to the usual capacitive coupling.
Fig.1 Linn Kivor Oktal, frequency response at -12dBFS, without emphasis, at 44.1kHz and 88.2kHz sample rates (top); and with emphasis, at 44.1kHz (bottom). (Right channel dashed, 0.5dB/vertical div.)
Fig.2 shows 1/3-octave spectral analyses of the Oktal's analog output while it decoded 16- and 24-bit data representing a dithered 1kHz tone at -90dBFS. The increase in bit depth results in only a 2dB drop in the noise floor, suggesting that the Oktal really offers 16-bit performance. Which is fine, considering that the Tunboks can send it only 16-bit data. But it does mean that the processor will not get the best from sources with greater resolution. Slight spuriae can be seen at 200Hz, 2kHz, and 7.5kHz. I'm reasonably confident that the two lower-frequency spikes are due to the Audio Precision System One test set, but the 7.5kHz peak appears to be characteristic of the Oktal. This peak can also be seen in the spectral analyses of the Oktal's output while it decoded "digital black" (fig.3). Note the dramatic increase in noise above 20kHz, due to the noise-shaping used to maximize the DAC's audioband resolution.
Fig.2 Linn Kivor Oktal, 1/3-octave spectrum of dithered 1kHz tone at -90dBFS, with noise and spuriae (from top to bottom): 16-bit data, 24-bit data (right channel dashed).
Fig.3 Linn Kivor Oktal, 1/3-octave spectrum of "digital black," with noise and spuriae (from top to bottom below 20kHz): 16-bit data, 24-bit data (right channel dashed).
Linearity error (fig.4) was very low down to -100dBFS, below which noise made its presence known. This noise can also be seen in the Oktal's reproduction of an undithered 1kHz sinewave at -90.31dBFS (fig.5), where it obscures what should be three discrete voltage levels. As expected from fig.2, no improvement in the waveshape could be seen when the word length was increased to 24 bits.
Fig.4 Linn Kivor Oktal, departure from linearity, 16-bit CD data (right channel dashed, 2dB/vertical div.).
Fig.5 Linn Kivor Oktal, waveform of undithered 1kHz sinewave at -90.31dBFS, 16-bit data.