The bane of digital audio may
continue to exercise the minds of designers responsible for CD players,
transports, outboard DACs, DAT, DCC and MD players alike, but it has also
provided a timely opportunity for yet another 'little black box' to clutter
the modern audio system. I refer, of course, to jitter and the recent emergence
of that handy device, the so-called 'jitter buster'.
Four of the breed are assembled
here, including the Digital Transmission Interface Plus (DTI+) from Audio
Alchemy, the Digital Interface Processor (DIP) from Monarchy Audio, Theta's
Timebase Linque Conditioner (TLC) and the new Pulsemaster from Trichord
Research. All four boxes are claimed to improve the quality of data passing
between a digital source and a digital input (typically an outboard DAC)
by reducing the incidence of jitter. Conveniently, all four also provide
a route to convert a Toslink optical to a coaxial digital output.
| Jitter
is the name given to any variation in the timing of a periodic event, such
as the binary code adopted in digital audio [diagram below]. The code is
represented by transitions or 'edges' (1) whose precise timing may be modulated
either by extraneous noise within the digital environment or, equally,
by the pattern of the data itself. This blurring of the edges (2) creates
an uncertainty in the position and timing of successive transitions (3)
when the data is sampled, say, within a DAC or at a digital interface.
For this test, a rig was assembled using a conventional CD player (with coaxial and optical digital outputs) and an outboard DAC whose digital interface offers little or no jitter attenuation. Data-pattern jitter was stimulated using a mixed-tone signal based on fractions of the 44.1kHz sample rate (first suggested by Julian Dunn at the 93rd AES Convention). Analysis, in the analogue domain, was completed using proprietary Vl (Virtual Instrument) software. Both Type I (phase-modulation) and Type 2 (phase and amplitude modulation) jitter are revealed as symmetrical and asymmetric sidebands, respectively, appearing either side of the final analogue signal (4): see the reference spectra. Importantly, this test provides a direct indication of the audible distortion caused by digital jitter and the effectiveness of each 'jitter buster' as it is placed between the transport and DAC. The total data-induced, peak-to-peak jitter generated via the CD player's coaxial digital output was computed to be 1584 picoseconds. The same test was repeated to determine the inherent jitter-level of the CD player's optical digital output, which amounted to 3241ps. The latter is about 10-times higher than ideal for a decent CD player and represents a hearty meal on which our 'jitter-busters' could dine. |
Effect of jitter on digital data 
Reference (source) jitter spectrum, coaxial 
Reference (source) jitter spectrum, coaxial |
JITTER
ANALYSIS: CD TRANSPORT-TO-DAC (REFERENCE)
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SUPPLIER
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| One
of many ‘little black boxes' in the Audio Alchemy range, the DTI Plus offers
inputs optical sources and provides a single coaxial output. Crystal's
evergreen CS8412 interface chip is run in 'test mode' with its free running
PLL (Phase Locked Loop) offering jitter rejection beyond its 20kHz bandwidth.
AA then provides a second stage of dejittering with a discrete crystal-based
PLL that has a claimed bandwidth of just 5Hz. Once activated, a yellow
LED lights up on the black fascia.
This master oscillator is used to re-clock the CS8412's digital output via a D-type flip-flop and buffer. In practice, however [Fig 1a and 1b], the common ground line shared by its AC-coupled Input and output aggravates a massive increase in fundamental data induced jitter (13) and random noise (14) at frequencies well beyond the 5Hz suggested in AA's literature. By way of comparison, a figure of 12304ps is nearly 100-times higher (worse) than that incurred by the best of today's budget CD players. In reality the bandwidth of the crystal PLL must be several hundred Hertz because, via the optical input at least, the suppression of jitter only starts to take hold beyond the 3rd (15) and 5th (16) harmonic sidebands. If only the DTI Plus had been a transformer-coupled unit, then this unhappy result may well have been avoided. |
Fig 1a: Audio Alchemy DTI Plus (coaxial) 
Fig 1b: Audio Alchemy DTI Plus (optical) |
JITTER
ANALYSIS: CD TRANSPORT-TO-DAC VIA AUDIO ALCHEMY DTI PLUS
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SUPPLIER
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| With
the trend towards fast, direct-coupled digital inputs and outputs becoming
more widespread the sluggish 19nsec risetime and non-standard 166ohm output
of Monarchy's DIP looks distinctly old-fashioned. Nevertheless Monarchy's
custom-made input and output coupling transformers successfully isolate
the digital source, receiving device and internal circuitry of the DIP
itself with result [Figs 2a and 2b] that fundamental data-induced jitter
is suppressed on both coaxial (17) and optical Inputs (18).
Between the decoupling transformers, a high speed receiver/transmitter chip feeds a professional 'repeater' that demodulates the incoming S/PDIF signal into respective clock and data lines which are then separately re-encoded and clocked-out via a highly stable oscillator. Jitter suppression would be better still if Monarchy was not using an internal PSU, for this contributes an extra 210 -250ps of jitter modulating both coaxial (19) and optical (20) output. |
Fig 2a: Monarchy DIP (coaxial)
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JITTER
ANALYSIS: CD TRANSPORT-TO-DAC VIA MONARCHY AUDIO DIP
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SUPPLIER
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| Built
into a small, folded steel box and fed from an encapsulated supply, Theta’s
TLC is an unfussy application of Crystal's CS8412 receiver chip. The TLC
uses a free running PLL within the CS8412 interface for clock recovery
and jitter suppression without a second crystal-based lock (as used by
Audio Alchemy, for example). The digital output is re-clocked via a D-type
flip-flop with extra 'signal conditioning provided by a high-speed hex
inverter—hence the TLC's high output level, clean waveshape and fast risetime.
This output is transformer coupled but, because Theta uses a 'stereo' dual-RCA phono socket (a cost-saving, presumably), the grounds of both coaxial input and outputs remain connected! Once again, as interference can circulate through a common ground plane there is actually a mild increase in data-reduced jitter from 1584ps to 1831ps. Via the optical input, this ground link is broken, thereby exposing the true potential of Theta's design; in this mode, jitter falls by 82% from 3241ps to 575ps. |
Fig 3a: Theta TLC (coaxial)
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JITTER
ANALYSIS: CD TRANSPORT-TO-DAC VIA THETA TLC
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SUPPLIER
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| This innovative
box of tricks may still employ the predictable Crystal CS8412 interface
chip with its bog standard 20-25kHz PLL but here the S/PDIF signal is also
clocked into a 20-bit sample-rate converter for, theoretically, a near-total
suppression of jitter. Both optical and coaxial inputs are provided though
a CS8402A transmitter services a single, direct-coupled coaxial output.
Via its coaxial link, the data induced jitter is reduced from 1584ps to just 167ps, though a variety of new jitter sidebands (24-26) now amount to some 1373ps. Similarly, data induced jitter is reduced from 3241ps to just 215ps via the optical input though some Trichord-specific sidebands (27-28) cause the total to rise to 1011ps. These 'new' jitter components arise from an intermodulation or 'beadng' between the asynchronous 11.2896MHz and 16.9344MHz clocks required by the AD1890 sample-rate converter. Fortunately, any sideband below 200Hz or so has less subjective impact than those at higher frequency. |
Fig 4a: Trichord Pulsemaster (coaxial)
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JITTER
ANALYSIS: CD TRANSPORT-TO-DAC VIA TRICHORD PULSEMASTER
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