ad1890jpz Analog Devices, Inc., ad1890jpz Datasheet - Page 15

• Manufacturer Part Number: ad1890jpz
• Description: Sampleport Stereo Asynchronous Sample Rate Converters
• Manufacturer: Analog Devices, Inc.

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Multiple ASRC Synchronization and Performance
Degradation
Multiple parallel AD1890/AD1891 ASRCs may be used in a
single system. Multiple AD1890/AD1891s can be “synchro-
nized” by simply sharing the same reset and MCLK lines, and
ensuring that all the ASRCs leave the reset state on the same
MCLK falling edge. No other provision is necessary since the
different AD1890/AD1891s will process samples identically if
they are presented with the same input and output clocks
(neglecting the effect of excessive clock skew on the PCB, as
well process variations between ASRCs which could cause dif-
ferent devices to trigger at slightly different times on excessively
slow rising or falling clock edges).
It is also likely that several AD1890/AD1891s could end up in a
serial cascade arrangement, either in a single systems design or
as the result of two or more systems, each using a single AD1890/
AD1891 in the signal path. The audio signal quality will be
degraded with each pass through an ASRC, though to a very
minor degree. The THD+N performance will degrade by 3 dB
with every doubling of the number of passes through an ASRC.
For example, the AD1890 THD+N specification of –106 dB (at
1 kHz) will rise to –103 dB if the signal makes two passes
through an ASRC. The overall system THD+N specification
will rise to –100 dB with four passes, and so on.
Clipping
Under certain rare input conditions, it is possible for the
AD1890/AD1891 ASRC to produce a clipped output sample.
This situation is best comprehended by employing the interpola-
tion/decimation model. If two consecutive samples happened to
have full-scale amplitudes (representing the peak of a full-scale
sine wave, for example), the interpolated sample (or samples)
between these two samples might have an amplitude greater
than full scale. As this is not possible, the AD1890/AD1891 will
compute a full-scale amplitude for the interpolated sample or
samples (see Figure 12). Clipping can also arise due to the
pre-echo and post-echo Gibbs phenomena of the FIR filter,
when presented with a full-scale step input. The result of this
erroneous or clipped output sample may be measured as an
extremely small decrease in headroom for transient signals.
FULL SCALE
AMPLITUDE
Figure 12. Clipped Output Sample
CORRECTLY INTERPOLATED SAMPLE
CLIPPED INTERPOLATED SAMPLE
TIME
–15–
Varispeed
It is also envisioned that the AD1890 will be used in varispeed
applications. The AD1890 and AD1891 SamplePorts are very
useful for converting an input data stream with a variable
sample rate (and therefore pitch characteristic) into an output
data stream with a constant sample rate.
Options for Sample Rate Conversion over a Wider Range
There are systems which require sample rate conversion over a
range which is wider than the 1:2 or 2:1 range provided by a
single AD1890 or AD1891, such as for “scrubbing” in digital
audio editors. There are at least two options in this situation.
The first is to use a programmable DSP chip to perform simple
integer ratio interpolation or decimation, and then use the
AD1890/AD1891 when this intermediate output sample fre-
quency is within the 1:2 or 2:1 range of the final desired output
sample frequency. The second is to use multiple AD1890/
AD1891 devices cascaded in series to achieve the required
sample rate range.
“Almost Synchronous” Operation
It is possible to apply input and output sample frequencies
which are very close (within a few Hz) or in fact synchronous
(LR_I and LR_O tied together). There is no performance pen-
alty when using the AD1890/AD1891 in “almost synchronous”
applications. Indeed, there is a very slight performance benefit
when the input and output sample clocks are synchronous since
the alias distortion components which arise from the non-infinite
stopband attenuation of the FIR filter will pile up exactly on top
of the sinusoidal frequency components of the input signal, and
will thus be masked.
System Mute
The mute function applies to both right and left channels on the
AD1890/AD1891. The user can include a system specific out-
put mute signal, while retaining the automatic mute feature of
the AD1890/AD1891 by using the circuit shown in Figure 13.
EXTERNAL SYSTEM MUTE
Figure 13. External Mute Circuit
ACTIVE HI
MUTE_O
16
AD1890/AD1891
AD1890/AD1891
MUTE_I
15