What is baudline?
Mystery Signal
Full Duplex DAQ comparison
E. Olson, Sep 28 2005

Test, measure, and characterize the full duplex performance of various data acquisition hardware and software drivers.  The device's ADC and DAC will be tested as a complete system in a loopback fashion.  The sample rate accuracy, frequency response, distortion, and quantization measurements will be analyzed and compared.

The DAQ device becomes the black box under test that is illustrated in the diagram to the right.  The input and output channels are connected together with an external loopback cable (or optionally with an internal "volume" mixer setting).  The AC coupling option is enabled to remove the effects of the DC offset from the distortion measurements.

The measurements are performed at all of the standard sample rates for a complete picture of the DAQ's capabilities and filter response.  The baudline test and measurement application notes below describe the procedural details:

The following device reports contain technical information, spectral response plots, detailed measurements, and analysis:

Sample Rate
The following table of measurements were made using the sample rate stability technique at the highest native sample rate supported by the DAQ device.  The in/out rate and 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.

device in rate out rate in error out error loop error
CS4236B 47972.865  47973.211  -565.313 PPM  -558.104 PPM  +0.0000 PPM 
SB Vibra16X 44086.273 44086.296 -311.270 PPM -310.748 PPM
SB16 PCI 48000.103  48000.103  +2.1396 PPM  +2.1396 PPM  +0.0000 PPM 
SB128 48003.136  48003.136  +65.3333 PPM  +65.3333 PPM  +0.0000 PPM 
SB Live! 48001.674 48005.204 +34.8750 PPM +108.417 PPM +0.0000 PPM
VIA 8235 47999.130  47999.245  -18.1208 PPM  -15.7271 PPM  +0.0000 PPM 
SiS 7012 48006.40  48006.42  +133.33 PPM  +132.75 PPM  +0.0000 PPM 
Maestro 2E 47185.71  48000.94  -16964.4 PPM  +19.5833 PPM  -16991.0 PPM 
iMic v0.06 48007.95  48007.94  +165.625 PPM  +165.417 PPM  ±2.4842 PPM 
iMic v3.00 48002.45  48008.97  +51.0417 PPM  +186.875 PPM  -130.653 PPM 
Edirol UA-25  44108.671  44108.671  +196.62 PPM  +196.62 PPM  +0.0000 PPM 

The devices without a loop error entry are half duplex.  The ± symbol represents a PPM range because the frequency was fluctuating. 

Most full duplex devices have a zero PPM loop error which signifies that their ADC and DAC clocks are in lockstep.  Large errors such as the ESS Maestro 2E's -17000 PPM are a sign of a major problem.  The SB Live's in / out error mismatch of 73 PPM represent a driver fragment problem.

A lot of internal details about the clock structure, sample rate conversion, and driver robustness can be learned from rate analysis.

The following table of measurements were made using the sine distortion technique at the highest native sample rate supported by the DAQ device.  This is a full duplex test that uses a loopback of the tone generator to measure the various SNR, THD, SINAD, ENOB, and SFDR metrics.  See the distortion measurement window for more information.  The stereo crosstalk column is a measure of channel bleed through that uses a sine wave channel and a silent channel as signal sources.

device rate SNR THD SINAD ENOB SFDR crosstalk
CS4236B 48000 +72.47 dB  -73.19 dB  +69.80 dB  +11.302 bits  +79.84 dB  -67.35 dB 
SB Vibra16X 44100i  +72.74 dB  -74.58 dB  +70.55 dB  +11.426 bits  +77.69 dB  -78.08 dB 
SB Vibra16X 44100o +75.24 dB -73.22 dB +71.10 dB +11.518 bits +76.95 dB -75.26 dB
SB16 PCI 48000 +78.60 dB -77.05 dB +74.74 dB +12.122 bits +79.87 dB -81.33 dB
SB128 48000 +78.88 dB  -80.66 dB  +76.67 dB  +12.442 bits  +87.61 dB  -77.51 dB 
SB Live! 48000 +78.63 dB  -77.99 dB  +75.29 dB  +12.213 bits  +80.55 dB  -81.17 dB 
VIA 8235 48000 +64.31 dB  -60.89 dB  +59.26 dB  +9.551 bits  +65.30 dB  -81.37 dB 
SiS 7012 48000 +69.84 dB  -70.95 dB  +67.35 dB  +10.894 bits  +72.69 dB  -86.33 dB 
Maestro 2E 48000  +17.70 dB  -30.52 dB  +17.48 dB  +2.611 bits  +25.62 dB  -66.47 dB 
Maestro 2E 48000o  +71.92 dB  -65.77 dB  +64.83 dB  +10.475 bits  +68.58 dB  -73.64 dB 
iMic v0.06 48000  +84.06 dB  -85.99 dB  +81.91 dB  +13.312 bits  +93.81 dB  -90.55 dB 
iMic v3.00 48000  +79.18 dB  -77.60 dB  +75.31 dB  +12.216 bits  +78.74 dB  -64.76 dB 
Edirol UA-25  44100a  +88.77 dB  -86.24 dB  +84.31 dB  +13.711 bits  +90.58 dB  -82.09 dB 
digital no dither +99.88 dB  -106.45 dB  +99.02 dB  +16.154 bits  +115.27 dB -inf.00 dB
digital dither +96.91 dB -103.54 dB +96.06 dB +15.662 bits +112.73 dB -inf.00 dB

The "i" and "o" suffixes in the sample rate column represent isolated testing of the input ADC (i) and the output DAC (o) components.  The digital device is the baseline level of performance for a lossless 16-bit channel.

Visually comparing the spectrogram sine sweeps and the average window filter response for different audio devices reveals many interesting details.  See the galleries below for an insightful view into the similarities and differences of the DAQ devices under test.


All of the tested devices fall far short from the theoretical ENOB of 16-bits. 

The quality and performance of different audio devices varies wildly.  Consumer sound cards have an incredible number of flaws and artifacts, some subtle and some not.  Performance as a function of sample rate is a major variable.  Quantization errors and inter channel delay also play a critical role. 

The most important lesson we learned in the creation of this application note is the value of conducting a thorough test and measurement procedure for all of a device's control parameters.  Many sound cards, that for years were thought to be top performers, had numerous flaws lurking under the surface.  The only way to uncover them was with a systematic and exhaustive search.

If you are an OEM please keep in mind that SigBlips can be contracted to help in your hardware and system development.  You can send us hardware to run the series of DAQ comparison tests on.  We can build an automated GoNoGo test system customized for your equipment manufacturing.  We can write or repair audio drivers.  Maximize performance and minimize design flaws by involving us in your development process. 

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