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Information
Sound Blaster Vibra16X ISA


vendor Creative Labs
product Sound Blaster 16
interface ISA
duplex half
channels 2
resolution 16 bits
max rate 44.1 Ksample/sec
chipset SB16 DSP v4.16
codec CTL1745
operating system Mandrake 10.1 Linux x86 2.6.8.1
driver sb16 ALSA 1.0.6
buffer size input 64 KB, output 64 KB
test date Sep 26 2005
notes Sound Blaster 16 at 0x240, irq 7, dma 0
include /etc/isapnp.conf

Disable "3D Enhancement" option with an external mixer program.


This card is part of the Full Duplex DAQ comparison survey.

The SB Vibra16X is a half duplex audio card and it cannot run in full duplex mode.  So a CS4236B was used as the test source and sink at a fixed 44100 sample rate.  The CS4236B is a very clean audio card and it has about a +11 PPM sample rate error at 44100.

 
Sample Rate
The sample rate on DAQ cards is not a fixed absolute constant.  Like time, it fluctuates, and it is difficult to measure accurately.  Sometimes there are relationships between the input and output sample rates that can reveal interesting details about the inner working machinery.

The following table of measurements use a technique described in the sample rate stability application note. The rate column is the sample rate value that the collection hardware is programmed to.  The in/out rate and in/out error columns are absolute measurements of the ADC / DAC clock. 

rate in rate out rate in error out error loop ADC loop DAC
4000 4977.4880 4977.4892 +244372. PPM +244372. PPM +244360. PPM +244359. PPM
5510 5510.7907 5510.7909 +143.503 PPM +143.539 PPM +133.818 PPM +133.100 PPM
8000 7999.5311 7999.5375 -58.6125 PPM -57.8125 PPM -67.7954 PPM -68.5200 PPM
11025 11021.576 11021.583 -310.567 PPM -309.932 PPM -319.762 PPM -320.430 PPM
12000 11910.420 11910.410 -7465.00 PPM -7465.83 PPM -7474.71 PPM -7475.40 PPM
16000 15999.067 15999.051 -58.3125 PPM -59.3125 PPM -67.7954 PPM -68.4950 PPM
22050 22043.167 22043.162 -309.887 PPM -310.113 PPM -319.738 PPM -320.426 PPM
24000 23998.620 23998.596 -57.5000 PPM -58.5000 PPM -67.9454 PPM -68.4870 PPM
32000 31998.120 31998.121 -58.7500 PPM -58.7188 PPM -67.8954 PPM -68.4800 PPM
44100 44086.273 44086.296 -311.270 PPM -310.748 PPM -319.728 PPM -320.419 PPM
48000  48025.908  48025.921  +539.750 PPM  +540.021 PPM  +533.785 PPM +524.527 PPM


All of the sample rates have a smooth convergence so the driver is behaving nicely.  The slight variation between the in error and out error measurements can be explained as thermal drifts since they had to be done at different times due to the Sound Blaster Vibra16X's half duplex nature.

The CS4236B was used as the source and the sink for the loop ADC and loop DAC measurements.  The CS4236B has a relative zero PPM loop error measurement so the ADC and DAC loop measurements can be compared.  It is interesting to note that the SB Vibra16X's relative loop errors are 11 PPM less than it's absolute rate errors.  The CS4236B's absolute rate error happens to be +11 PPM.

The 8000, 16000, 24000, and 32000 sample rates have in / out errors of -58 PPM.  Another interesting grouping is the 11025, 22050, and 44100 sample rates which have in / out errors of -310 PPM. 

The SB Vibra16X does not natively support the 48000 sample rate so the +540 PPM error is more of an insight into ALSA's sample rate converter. 

Something is wrong with the 4000 and 12000 sample rates which have respective sample rate errors of +244372 and -7465 PPM.  The 5510 rate is also an oddball with it's +143 PPM error which is probably caused by 5510 not being 5512.5 which is exactly half of 11025.

 
Frequency Domain
The sound card's input and output jacks are connected with a short external cable and run in full duplex mode.  This is a loopback test and baudline's tone generator is the signal source.  Distortion, noise floor, filter response, and inter channel crosstalk are the frequency domain measurements of interest in this section. 

The signal test sources are a pure sine wave, a linear sine sweep, and WGN.  The sine wave is used for the distortions and crosstalk measurements.  The linear sine sweep and WGN are used for the filter characterization measurement.  Both are an application of the swept sine vs. WGN technique and are equivalent measures of the frequency response. 

Since spectral performance is a function of sample rate, each of the sound card's native rates will be tested.  The highest sample rate is usually the cleanest and this is advantageous because it allows the isolated testing of the ADC and the DAC.  The matched, source, and sink sample rate combinations are described below.

source
Since the Vibra16X is only a half duplex device the CS4236B at 44100 was used for test signal capture purposes.  The Vibra16X is the source while the CS4236B is the sink.  This combination tests the performance of the Vibra16X's DAC.  The linear sine sweep signal in the left spectrogram display characterizes the DAC filter response.  The position of the pass-band and the stop-band filter transition is defined by the Nyquist frequency of the DAC.  The noise floor (purple) is the crosstalk channel at the 44100 sample rate and the Average collection of a silent channel at all the other sample rates..

sink
Since the Vibra16X operates only in half duplex mode the CS4236B at 44100 was used for test signal generation.  The CS4236B is the source while the Vibra16X is the sink.  This combination tests the performance of the Vibra16X's ADC.  The linear sine sweep signal in the right spectrogram display and the orange curve in the Average window below it characterize the ADC filter response.  The position of both the pass-band and the stop-band filter transition is defined by time in the spectrogram and by folded frequency in the Average window.  The orange Average curve represents the pass-band while the cyan curve is a folded representation of the stop-band ADC filter response.  The noise floor (purple) is the crosstalk channel.

The naming convention for the columns below is (DAC -> ADC) where DAC represents the source sample rate and ADC represents the sink sample rate. 


source (DAC)
sink (ADC)
4000 -> 44100 44100 -> 4000


5510 -> 44100 44100 -> 5510


8000 -> 44100 44100 -> 8000


11025 -> 44100 44100 -> 11025


12000 -> 44100 44100 -> 12000


16000 -> 44100 44100 -> 16000


22050 -> 44100 44100 -> 22050


24000 -> 44100 44100 -> 24000


32000 -> 44100 44100 -> 32000


44100 -> 44100 44100 -> 44100


48000 -> 44100 44100 -> 48000

Something is wrong with the 48000 sample rate.



distortion (ADC)
The following table of measurements were made using the technique described in the sine distortion application note.  It is a full duplex test that uses a loopback of the tone generator to measure the various distortion parameters.  The stereo crosstalk column is a measure of channel leakage that uses a sine wave channel and a silent channel as the signal sources.

rate SNR THD SINAD ENOB SFDR crosstalk
4000 +75.48 dB -72.36 dB +70.64 dB +11.440 bits +74.58 dB -85.64 dB
5510 +75.04 dB -75.41 dB +72.21 dB +11.701 bits +78.30 dB -85.70 dB
8000 +73.81 dB -77.09 dB +72.14 dB +11.689 bits +80.09 dB -84.72 dB
11025 +73.78 dB -78.30 dB +72.47 dB +11.745 bits +81.21 dB -83.86 dB
12000 +72.91 dB -77.70 dB +71.66 dB +11.611 bits +81.17 dB -84.39 dB
16000 +73.27 dB -78.06 dB +72.02 dB +11.671 bits +81.48 dB -83.22 dB
22050 +73.07 dB -76.75 dB +71.52 dB +11.587 bits +80.80 dB -81.27 dB
24000 +72.79 dB -76.64 dB +71.29 dB +11.549 bits +81.14 dB -81.40 dB
32000 +72.05 dB -76.91 dB +70.82 dB +11.472 bits +81.11 dB -79.71 dB
44100  +72.74 dB  -74.58 dB  +70.55 dB  +11.426 bits  +77.69 dB  -78.08 dB 
44100o  +75.24 dB -73.22 dB +71.10 dB +11.518 bits +76.95 dB -75.26 dB
48000 +47.26 dB -50.02 dB +45.42 dB +7.251 bits +54.55 dB -77.19 dB *

The "*" symbol signifies the distortion metrics for that particular sample rate are invalid due to spectral damage caused by wideband noise pulses.

All of the distortion measurements are from the sink(ADC) column except for the "44100o" sample rate which is from the source(DAC) column.

The 48000 sample rate has poor distortion measurements because that particular rate is being synthesized by ALSA.

The ADC's and DAC's have -60 dB attenuation at all sample rates except 48000.  The source and sink filter shapes look identical too.  The Vibra16X's application of filters is extremely consistent.

 
Quantization
A white Gaussian noise signal source was generated and captured in full duplex loopback fashion at each of the standard sample rates.  The Histogram plots below show a unique sample distribution that is dependent on sample rate.

4000 ... 48000


Nice clean looking Gaussian shaped histogram.  The left and right channels have different DC offsets.

 
Channel Delay
A sine wave signal was generated and captured in full duplex loopback mode.  The time domain response was observed with the Waveform window where the green curve represents the left channel and the purple curve represents the right channel.  Only a cyan sine curve is visible below because the left and right channel match up perfectly.

4000 ... 48000


The SB Vibra16X ISA card a zero sample inter channel delay at all sample rates.

 
Analysis
The large sample rate estimation errors do not have any effect on the spectrogram sweeps or the distortion measurements.  Performance is consistent across all sample rates except for the 48000 rate.

 
Conclusion
The Sound Blaster Vibra16X has very good performance for an old half duplex ISA card.  The card doesn't natively support the 48000 sample rate and it is best to avoid using it.  The Vibra16X has large sample rate accuracy errors at the 4000 and 12000 rates and it is best to avoid using those too.

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