baudline
Home
News
What is baudline?
Screenshots
Download
FAQ
Manual
Search
Solutions
Mystery Signal
Contact
Information
VIA 8235


vendor Via Technologies
product VT8235
interface chip on ECS P4VXASD+ motherboard
duplex full
channels 2
resolution 16 bits
max rate 48 Ksample/sec
chipset VIA8233 rev 50
codec Realtek ALC101
operating system Red Hat 7.3 Linux x86 2.6.7
driver via82xx ALSA 1.0.4
buffer size input 64 KB, output 64 KB
test date Sep 7 2005
notes Very strong DC offset of -35 dB on the right channel.  


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

 
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.  The loop error column uses a tone generator loopback method for a high accuracy measurement of the relative difference between the ADC and the DAC clocks.  The three error PPM columns are theoretically related by the formula: "in_error - out_error = loop_error"

rate in rate out rate in error out error loop error
4000 4010.3834 4010.3880 +2595.85 PPM +2597.00 PPM +144.295 PPM *
5510 5505.3010 5505.3037 -852.813 PPM -852.323 PPM -170.005 PPM *
8000 7999.8866 7999.8932 -14.1750 PPM -13.3500 PPM +247.488 PPM *
11025 11010.5997 11010.6039 -1306.15 PPM -1305.77 PPM -35.3544 PPM *
12000 11999.7891 11999.8130 -17.5750 PPM -15.5833 PPM +0.0000 PPM
16000 15999.7376 15999.7508 -16.4000 PPM -15.5750 PPM -50.9133 PPM *
22050 22021.1385 22021.1482 -1308.91 PPM -1308.47 PPM -22.5884 PPM *
24000 23999.6052 23999.6165 -16.4500 PPM -15.9792 PPM +0.0000 PPM
32000 31999.4797 31999.4876 -16.2594 PPM -16.0125 PPM +2.5234 PPM *
44100 44042.3070 44042.3116 -1308.23 PPM  -1308.13 PPM  +2.3241 PPM  *
48000  47999.1302  47999.2451  -18.1208 PPM  -15.7271 PPM  +0.0000 PPM 

The "*" symbol signifies severe spectral damage such that the loop error measurements might be off by an order of magnitude.


The 12000, 24000, and 48000 sample rates all have a zero PPM loop error measurement and an absolute rate offset of about -16 PPM.  This means that the ADC and DAC clocks are in lockstep at those particular sample rates.  These are also the only rates that didn't suffer from severe spectral damage.

The 11025, 22050, and 44100 sample rates have in/out errors of about -1308 PPM.  The 5510 rate is roughly two thirds that at -852 PPM.  The 4000 rate, which is not related at all, has an error of almost three times -1308 with a +2600 PPM error.  Not sure how the error ratios for the 4000 and 5510 rates are connected but a common algorithmic flaw is at work.

Below is the plot for the 22050 sample rate which shows a nice clean convergence.  All of the VIA 8235's sample rates display this respective shape and behavior.  This signifies the via82xx driver is handling the fragment interrupts properly.


 
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.

matched
The input and output sample rates are the same.  This combination tests the performance of both the ADC and the DAC in a matched mode of operation.  The linear sine sweep signal in the left spectrogram display and the WGN (orange) in the Average window characterize the in-band filter response.  The sine wave (green) in the Average window is used for distortion and crosstalk measurements.  The sine leakage (purple) is used for crosstalk measurement

source
The sample rate of the input (sink) is the card's highest clean rate.  This combination tests the performance of the DAC.  The linear sine sweep signal in the middle 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 Average collection of a silent channel.

sink
The sample rate of the output (source) is the card's highest clean rate.  This combination tests the performance of the ADC.  The linear sine sweep signal in the rightmost 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 Average collection of a silent 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. 


matched
source (DAC)
sink (ADC)
4000 -> 4000 4000 -> 48000 48000 -> 4000

The wideband pulses in both the 4000 matched and source spectrogram sweeps have a 0.382 second period (1528 samples).

5510 -> 5510 5510 -> 48000 48000 -> 5510

The wideband pulses in both the 5510 matched and source spectrogram sweeps have a 0.410 second period (2259 samples).

8000 -> 8000 8000 -> 48000 48000 -> 8000

The wideband pulses in both the 8000 matched and source spectrogram sweeps have a 0.128 second period (1024 samples).

11025 -> 11025 11025 -> 48000 48000 -> 11025

The wideband pulses in both the 11025 matched and source spectrogram sweeps have a 0.205 second period (2260 samples).

12000 -> 12000 12000 -> 48000 48000 -> 12000


16000 -> 16000 16000 -> 48000 48000 -> 16000

The wideband pulses in both the 16000 matched and source spectrogram sweeps but the period is too fast to be able to measure accurately.

22050 -> 22050 22050 -> 48000 48000 -> 22050


24000 -> 24000 24000 -> 48000 48000 -> 24000


32000 -> 32000 32000 -> 48000 48000 -> 32000

The wideband pulses in the source spectrogram sweeps have a 0.128 second period.

44100 -> 44100 44100 -> 48000 48000 -> 44100


48000 -> 48000  
 

The 48000 filter looks like it doesn't have any high frequency roll-off.



distortion
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 +39.69 dB -44.87 dB +38.54 dB +6.109 bits +44.05 dB -81.27 dB
5510 +40.21 dB -48.50 dB +39.61 dB +6.287 bits +46.21 dB -81.55 dB
8000 +37.49 dB -46.93 dB +37.02 dB +5.857 bits +42.30 dB -82.16 dB
11025 +33.15 dB -48.23 dB +33.02 dB +5.191 bits +38.22 dB -82.52 dB
12000 +63.72 dB -63.49 dB +60.59 dB +9.772 bits +69.92 dB -81.73 dB
16000 +34.76 dB -47.20 dB +34.52 dB +5.441 bits +40.32 dB -80.75 dB
22050 +25.86 dB -39.89 dB +25.70 dB +3.975 bits +31.60 dB -82.15 dB
24000 +63.88 dB -61.87 dB +59.75 dB +9.632 bits +66.84 dB -80.61 dB
32000 +23.60 dB -41.95 dB +23.54 dB +3.617 bits +23.62 dB -80.42 dB
44100 +18.48 dB -29.20 dB +18.13 dB +2.718 bits +22.86 dB -81.04 dB
48000  +64.31 dB  -60.89 dB  +59.26 dB  +9.551 bits  +65.30 dB  -81.37 dB 


The distortion measurements are a fair numeric representation of the test signals that are depicted in the spectrum and spectrogram images above.

All the rates except 12000, 24000, and 48000 suffer from some major spectral flaws.  The flaws are in the form of wideband pulse, crosshatch aliasing, folded aliasing, or a combination of all three.  The 12000, 24000, and 48000 have the best distortion values.

The filters on the playback DAC side are very poor with minimal rejection.  The filters on the record ADC side are nonexistent.

The spectrogram sweeps and the average spectrum plots look surprisingly similar to the SiS 7012 audio device which also uses a Realtek codec.

 
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.

The purple and green Gaussian histogram shapes are separated because of the large inter channel difference of DC offsets.

12000, 24000, 48000

Every even sample value is zero as if the LSB is fixed in the off position.  This transforms a 16 bit ADC into a 15 bit ADC.  The histogram spikes have a very interesting shape and are very periodic with a two high then two low cadence pattern.  Realtek claims the ALC101 codec is an 18-bit device.

4000, 5510, 8000, 11025, 16000

As the sample rate increases the solid histogram hump also increases in size.  The effect of the zeroed LSB is clearly evident in this family of histogram plots.  The same basic shape and spike pattern as in the previous histogram.

22050, 32000, 44100

Very similar to the previous histogram but the solid lobe section is larger and the original histogram shape is harder to discern.

 
Channel Delay
The VIA 8235 chipset has a zero sample inter channel delay.

 
Analysis
The 12000, 24000, and 48000 rates have a zero PPM sample rate loop error, clean spectrum / spectrogram sweeps, low distortion, and the same zeroed LSB histogram plots.  48000 is the native rate and all the other sample rates are derived from that.  The 12000 and 24000 are implemented as simple decimate by 4 and by 2 without any filtering.  On the DAC side interpolation is done with a low order filter for only about -20 dB of rejection.

The large sample rate PPM in/out errors seem to correlate well with the crosshatch aliasing found on the sink ADC side for the 4000, 5510, 11025, 22050, and 44100 rates.  Not sure what the DSP connection is but it probably has to do with the flawed algorithm that is performing the sample rate conversion.

The zeroed LSB equivalent 15-bit converters probably aren't helping the +9.55 bit ENOB measurement.

 
Conclusion
The filters on the playback DAC side are very poor with minimal rejection.  The filters on the record ADC side are nonexistent.

Only use the 48000 sample rate with the VIA 8235 chipset.  All of the other sample rates suffer from severe spectral damage.  Audio playback at 44100 is not recommended with this chip.

Copyright © 2005 SigBlips.com - group - blog - site map