home entertainment 2007

analog output. Its S/PDIF and DAC
clocks are driven by a 24.5297MHz
crystal oscillator.
The SoundBridge handles most digital
music formats, including MP3,
AAC, AIF, WAV, ALC, Ogg-Vorbis,
Windows Media, and FLAC. When
your music-server software sends compressed
music files to the SoundBridge,
it decodes them in real time to PCM
before sending them on to its DAC
and S/PDIF outputs.
measurements, continued
SoundBridge produced reasonably low levels of spuriae.
Fig.6 was taken with the Roku decoding data representing
a full-scale 1kHz tone into a fairly low impedance of 8k
ohms. The volume control was set to “88%” (–7dB) for
this measurement, because I wanted to make sure I was
not overdriving the unit’s output. The THD was a very low
0.006% from both channels, with the third harmonic the
highest in level at –85dB. But note the picket fence of
spurious tones in the Roku’s output. Yes, these are all
almost 100dB below the level of the signal, but they
shouldn’t be there at all. Suspecting that the volume control
was to blame, I repeated the test with it set to
“100%,” but other than the increase in volume, the result
was identical. The SoundBridge’s analog output was less
well behaved at low frequencies: even into 100k ohms, a
full-scale 50Hz tone was accompanied by the third harmonic
at –62dB and the fifth at –74dB (fig.7). The distortion
components rose considerably with the SoundBridge
driving a low 600-ohm load (not shown). This player is
not comfortable driving low frequencies into impedances
much below 10k ohms, in my opinion.
Intermodulation distortion with an equal mix of 19kHz
and 20kHz tones was respectably low (fig.8), but again,
the noise floor in this graph is obscured by spurious tones.
Finally, when tested for its rejection of word-clock jitter on
the incoming audio data using the Miller Jitter Analyzer,
the SoundBridge produced a very high 3 nanoseconds
peak–peak of jitter. Fig.9 is a narrowband spectrum of the
DAC’s analog output while it decoded 44.1kHz data repre-
Fig.6 Roku SoundBridge M1001, spectrum of 1kHz sinewave at 0dBFS into
8k ohms, volume control at “88%” (–7dB) (linear frequency scale).
Why the SoundBridge Plays
for Sure
Under its Plays for Sure protocol, and
as explained on its website, 3 Microsoft
licenses to and permits distribution and
playback of DRM-protected musical
material by only two network music
players—those made by Roku and D-
Link. This means that the SoundBridge
3 www.microsoft.com/windows/windowsmedia/for
pros/drm/sdksandversions.aspx#compare.
can play protected music files downloaded,
for example, from
Napster.com, Rhapsody.com, MSN_
Music_Now.com, and WalMart.com.
When you click on the protected
song’s title with the Roku’s remote, the
SoundBridge displays the message
“ACQUIRING LICENSE,” signifying that
it is matching its code to that of the
DRM-wrapped music file. That done,
the player can then receive the protected
file over a home network, though
senting a high-level tone at exactly one-quarter the sample
rate, over which had been laid the LSB toggling on
and off at exactly 1/192 the sample rate. Data-related
sidebands are absent, but: the noise floor is 15dB higher
than with the best 16-bit DACs I have measured; the central
peak representing the 11.025kHz tone is broadened at
its base, presumably due to low-frequency random jitter;
and a plethora of sidebands can be seen, either powersupply–related
(brown numeric markers) or of unknown
origin (purple).
LG did note that he was not impressed by the sound of
the Roku SoundBridge’s analog outputs, and so ended up
using its S/PDIF output to feed digital audio data to an
outboard D/A processor, which should have bypassed all
the problems with the implementation of the Sound-
Bridge’s analog circuitry. To check that this was indeed the
case, I connected first its TosLink output, then its coaxial
digital output to the Musical Fidelity X-24K that serves as
my test lab’s utility DAC. The X-24K has excellent jitter
rejection (see www.stereophile.com/digitalproces
sors/825/index7.html), producing just 240 picoseconds
peak–peak when fed from the soundcard in my PC by a
TosLink connection. However, the TosLink connection from
the Roku gave a high 2083ps of jitter (2.08ns) in the
Musical Fidelity’s output, with the spectral analysis (fig.10)
indicating a series of sidebands spaced at 150Hz and its
harmonics to the sides of the spectral line representing
the 11.025kHz tone. Note also the regular series of spurious
tones in this graph, indicated with green numeric
Fig.7 Roku SoundBridge M1001, spectrum of 50Hz sinewave at 0dBFS into
100k ohms, volume control at “100%” (linear frequency scale).
70 www.Stereophile.com, May 2007