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This page has examples from Microsoft Windows.
There is no fundamental difference between operating
systems, they all have the same problems.
An SDR hardware such as WSE, Softrock, IQ+ or Soft66 has a maximum signal level. It could be 4 V p-p (Softrock) 8V p-p (IQ+) or 16V p-p (WSE) or something else. The soundcard used should saturate close to the maximum level that the hardware can supply and it should have a good dynamic range (LOW NOISE FLOOR) when set to the required gain. To evaluate a soundcard, feed a signal into it and increase the signal level until saturation is observed in the spectrum. Saturation may or may not be at the full range of the soundcard. Compare the signal level at the point of saturation with the noise power in a fixed bandwidth. Do the measurements at different settings for the soundcard to produce a table like table 1 which is for SB Live! External USB running in I/Q mode at 96 kHz, 24 bit under Windows 7. |
Level Saturation point Noise in 1kHz Dynamic range control (V rms) (dB) (dB) (dBc/Hz) 100 1.259 -0.3 -107.8 137.8 90 1.435 0.0 -108.7 138.7 80 1.600 0.0 -109.7 139.7 70 1.840 0.0 -110.9 140.9 60 2.136 0.0 -112.2 142.2 50 2.240 -1.2 -114.0 142.8 40 2.240 -3.2 -115.8 142.6 30 2.240 -5.7 -118.4 142.7 25 2.240 -7.2 -119.8 142.6 20 2.240 -9.2 -121.7 142.5 15 2.240 -11.7 -124.4 142.7 10 2.240 -15.2 -137.8 142.6 5 2.240 -21.2 -133.8 142.6 |
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Table 1. SB Live! External USB under Windows 7. The dB
scales are vs the nominal point of saturation. This soundcard
needs hardware that can supply 2.2V rms (=6.2 V p-p) for full
dynamic range.
The measurements are made with an IQ+ that provides a stereo output. One can also use a tone generator and evaluate a single audio channel. One would then observe 3 dB lower dynamic range values which can be compensated for. The noise floor is measured with a grounded input. The noise floor varies from time to time. It looks like there are two volume controls and that a specific gain can be set with different combinations of the two gain controls. I have not been able to find any way to set the controls independently. Maybe some software is missing. Another soundcard from Creative, the SB audigy, behaves similarly. The data is shown in figure 2 which is for 48 kHz sampling rate with 24 bits under Windows 7. |
Level Saturation point Noise in 1kHz Dynamic range control (V rms) (dB) (dB) (dBc/Hz) 100 1.071 0.0 -107.5 137.5 90 1.205 0.0 -108.5 138.5 80 1.351 0.0 -109.6 139.6 70 1.517 0.0 -110.6 140.6 60 1.805 0.0 -112.1 142.1 50 2.142 0.0 -113.6 143.6 45 2.142 -1.0 -114.5 143.5 40 2.142 -2.0 -115.5 143.5 35 2.142 -3.0 -116.6 143.6 30 2.142 -4.5 -118.0 143.5 25 2.142 -6.0 -119.5 143.5 20 2.142 -8.0 -121.6 143.6 |
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Table 2. SB Audigy under Windows 7.
The dB scales are vs the nominal point of saturation.
This soundcard needs hardware that can supply 2.2V rms (=6.2 V p-p) for full
dynamic range.
The motherboard soundcard of my D5400XS Intel motherboard, STAC2274D high definition audio from Sigmatel (IDT) behaves differently. See table 3 which shows performance under Windows 7 when sampling 24 bits at 48 kHz. |
Level Saturation point Noise in 1kHz Dynamic range control (V rms) (dB) (dB) (dBc/Hz) 85-100 0.134 -1.2 -101.9 130.7 70-84 0.169 -1.2 -103.0 131.8 57-69 0.199 -1.2 -104.1 132.9 45-56 0.238 -1.2 -105.3 134.1 35-44 0.276 -1.2 -106.3 135.1 29-35 0.327 -1.2 -107.2 136.0 22-28 0.386 -1.2 -108.0 136.8 16-21 0.456 -1.2 -108.7 137.5 12-15 0.538 -1.2 -109.0 137.8 8-11 0.634 -1.2 -109.3 138.1 4-7 0.755 -1.2 -109.3 138.1 2-3 0.908 -1.2 -109.3 138.1 0-1 1.063 -1.2 -110.6 139.4 |
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Table 3. Sigmatel STAC2274D under Windows 7.
The dB scales are vs the nominal point of saturation.
This soundcard needs hardware that can supply 1 V rms (=2.8 V p-p) for full
dynamic range.
If your hardware is designed to be operated at maximum 0.5 V RMS audio output, The Sigmatel soundcard would allow a dynamic range of about 137.5 dB. The Soundblaster soundcards would have to be operated at their maximum setting and yet 0.5 V rms would be 6 dB or more below saturation for a dynamic range of 132 and 134 dB respectively. The Soundblaster Audigy can do 96 kHz/24 bit under Linux, but only on old versions of ALSA, I.e. under Ubuntu 6.10. Modern alsa drivers for Audigy do not work at all. (At least not with Linrad.) There should be drivers for Windows that allow the Audigy to run at 96 kHz but I have faild to find them alternatively failed to configure them properly. The Sigmatel High Definition Audio can do 192 kHz/24 bit under Linux. There are two volume controls, Mux and Capture. With both set to minimum saturation is at 0.95 V rms with a signal that is 3.3 dB below full range. The noise floor varies with the frequency. The dynamic range is 139 dB at 5 kHz but only 124 dB at 85 kHz. The Delta44 from M-Audio works well under Windows XP, but only with the old driver version 5.10.0.5047 (Installer available on the Internet as Delta_WDM_5.10.00.0048a.exe) Modern drivers for Delta 44 do not work correctly under XP or any other Windows version and there is no driver that works correctly undet Vista or Windows 7 according to my findings: Testing soundcard compatibility with modern computers. The Delta 44 has 3 gain settings with characteristics as shown in table 4. |
Level Saturation point Noise in 1kHz Dynamic range control (V rms) (dB) (dB) (dBc/Hz) +4dBV 4.14 0.0 -118.0 148.0 Consumer 2.07 0.0 -116.3 146.3 -10dBV 1.041 0.0 -112.7 142.7 |
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Table 4. Delta 44 under Windows XP with the 5.10.0.5047 driver.
The dB scales are vs the nominal point of saturation.
This soundcard needs hardware that can supply 4 V rms (=11.3 V p-p)
for full dynamic range.
There are a couple of design errors on the Delta 44 . One can correct them and also remove the input amplifier as described here. A modified Delta 44 has the characteristics shown in table 5. |
Level Saturation point Noise in 1kHz Dynamic range control (V rms) (dB) (dB) (dBc/Hz) +4dBV 5.0 0.0 -121.0 151.0 Consumer 2.48 0.0 -119.6 149.6 -10dBV 1.25 0.0 -118.2 148.2 |
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Table 5. The modified Delta 44 under Windows XP with
the 5.10.0.5047 driver.
The dB scales are vs the nominal point of saturation.
This soundcard needs hardware that can supply 5 V rms (=14.2 V p-p)
for full dynamic range.
At the -10dBV setting it provides the same performance, 148 dBc
for a 12 dB weaker signal compared to the unmodified card.
The unmodified card has a 5 dB lower dynamic range with the
-10dBV setting.
The modified Delta 44 has a 12 dB resistive voltage divider on the input and it can easily be adapted to levels down to 1.25V with a dynamic range of 151 dBc/Hz. The optimum gain setting for a soundcard can be deduced from the tables above or the corresponding table for other soundcards. A SDR unit like the Softrock that uses 5V DC for the final OP-amp might be useful for an output level up to 1.25V rms (=3.5 V p-p.) At higher amplitudes the harmonics become much stronger. Such a unit would give the dynamic ranges shown in table 6 if the unit itself produced no noise. |
Soundcard Gain setting Ideal dynamic range SB Live! External 100 138 dBc/Hz ** SB Audigy 85 139 dBc/Hz Sigmatel STAC2274D 0 139 dBc/Hz Delta 44 (original) -10dBV 143 dBc/Hz Delta 44 (mod) -10dBV 148 dBc/Hz Delta 44 (mod 2) +4dBV 151 dBc/Hz |
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Table 6. The optimum gain settings for use with a Softrock
and similar units that use 5V DC for the final OP-amps.
Delta44 mod 2 is without the resistive voltage divider on the input.
**Note that there is a driver, WASAPI, under Windows Vista that provides 142 dBc/Hz A softrock is at about 148 dBc/Hz when used with an ideal soundcard. Only the Delta 44 (mod 2) would take advantage of its full performance while the other soundcards would degrade by 6 dB or more. |