The J-Test, where J stands for jitter, was developed in the mid-1990s by the late Julian Dunn of PrismSound to investigate the jitter rejection of digital datalinks in which the clock is embedded in the audio data: the balanced AES/EBU or AES3 link, for example, or the unbalanced S/PDIF link. The test signal comprises a high-level tone at exactly one-quarter the sample frequency, or Fs/4, to which is added a squarewave at the level of the least significant bit (LSB), at Fs/192. With the twos-complement PCM encoding used by CD data, this low-level squarewave exercises all the bit transitions simultaneously, which is very much the worst case for stimulating jitter. The high-level, high-frequency tone is thus modulated by the jitter and sidebands spaced at the frequency of the Fs/192 squarewave, and its harmonics will appear in the reconstructed analog signal's noise floor.

With CD data, the high-level tone has a frequency of 44.1/4kHz = 11.025kHz, while the squarewave has a frequency of 44,100/192 = 229.6875Hz. The digital-domain waveform of this signal is shown in fig.1, with the data values shown as red dots and the analog waveform that would be reconstructed by the anti-aliasing low-pass filter as the blue trace. Something that is little appreciated is that because both test signals are exact even-integer fractions of the sample rate, there is therefore no quantizing noise/distortion. Any spuriae that appear in the reconstructed analog noise floor are due to the behavior of the device under test. Fig.2 shows the spectrum of the J-Test signal as decoded by a perfect 16-bit device (red and blue traces). Note that between the spectral lines, there is no noise at all above the –150dB floor of this graph.

Fig.1 Waveform of 16-bit Dunn analytical J-Test signal, fundamental frequency 11.025kHz, sample rate 44.1kHz.

Fig.2 Spectrum of 16-bit signal shown in fig.1. Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz (left channel blue, right red).

However, there is a problem with the frequencies chosen for the J-Test, as the Fs/4 tone occurs exactly midway between two of the odd-order harmonics of the Fs/192 LSB squarewave. Thus, once the jitter gets low enough, the true sidebands (with 44.1kHz data) at ±229.6875Hz (and its multiples) cannot be distinguished from the naturally occurring odd harmonics at 10,705.312Hz and 11,254.687Hz, etc. This is why I refer in my reviews to the "residual" level of the data-related products.