Sound card based multimeter Scope
Results of SB analysis
Possibility of DC input solution
The final solution
Notes for construction
PC sound card, standard component of practically all current personal computers,
consists, in minimum, of two parts: mixer A/D converter
Those parts are usually doubled as all sound cards are two channels - e. g. stereo devices.
They may contain other parts, but from the point of view of analog to digital (A/D)
signal processing they may not be as important.
Mixer is device joining analog signals from different signal sources into one which is delivered to A/D conversion. Important feature of current mixers is full software control of gain and level of all channels.
A/D converter is usually high precision 16-bit analog to digital converter
with maximum sampling rate 44.1 kHz, or 48 kHz.
All (4 ;-) sound cards we've tested and analyzed have the same feature
- all their line-in inputs (and others too) are insulated from DC input by condenser.
The reason is, clearly, to set zero level of processed sound signal stable and close to zero.
This is, however, a strong limitation for other use, for example for DC measurements.
The situation, fortunately, is not so bad as it seems...
■ SB 분석 결과
We've analyzed, with a good magnifying glass and ohmmeter,
LINE IN inputs of four different sound cards - three from Creative Labs, one from Manli:
SB16 (PCI), model CT4810(?),
AWE-64 (IDE), model 4520,
Audio PCI 5000 (PCI), with chip ES1371,
Manli CMI8738SX (PCI)
Inputs of SB16, Audio PCI5000, AWE-64 and CMI8738SX, respectively, are on the Fig. 1.
Fig. 1: SB16, Audio PCI5000, AWE-64 and CMI8738SX LINE IN inputs.
As it can be seen, three inputs are very simple.
Input of AWE-64 is a bit more complicated.
It contains standard FET operating amplifier,
but not condenser at the input.
The condenser is at the output, however...
All four sound cards have DC default level at mixer input significantly high.
The values displayed don't reflect reality.
It was found, that it is close to 2.5V. Direct connection of external DC is not applicable.
Is there a solution?
■ 직류입력 solution 가능성
There exists well known application of operating amplifier - differential amplifier.
출력은 다음 공식을 따른다
The analysis of the formula shows, that at the output there can be defined DC level even
in the case that U2 will be zero - provided appropriate signal will be delivered as U1.
A brief inspection of Fig. 1 shows, that AWE-64 already has operational amplifier.
Good new! It is candidate No. 1. Assumed changes will be negligible.
What is needed:
disconnect noninverting input from ground,
make tunable source of U1, one for both channels should be enough,
with a piece of wire make a shortage between condenser pins,
with running PC tune U1 to get appropriate DC level.
To make small board with operating amplifier and a few other components is possible too.
It is only solution for the rest three sound cards (and the majority of others).
■ 결과의 회로는 Fig. 3이다.
Fig. 3: AWE-64 input modification.
It works fine. With the device we did a few measurements.
It was found that the maximum sensitivity is approximately +/- 100 mV.
Based on +/- 5 V power it can be assumed that maximum input DC signal will be approximately +/- 2.5 V.
Higher voltages must be decreased by hardware divider.
With operational amplifier on small PCB we checked SB16 and CMI8738SX too.
The final version had been made with CMI8738SX Manli sound card.
Only reason for selection was that it was cheapest and is currently available.
Additional to Fig. 3 the schematics contais -5V chip, as sound card has none.
The rest are a bit older models. The results of all sound cards were comparable.
The solution is not limited to AWE-64 and comparables.
It seems any sound card can be modified provided small PCB with operational amplifier will be added.
At the begining AWE-64 was chosen only due to fact it already has the (pre)amplifier.
However for the final solution it was not suitable - in between mother board
of home computer changed and ISA slot gone...
■ 최종 결과
To test, and even to use the device, one need some useful program.
As a first choice we decided to get Konstantin Zeldovich's Winscope.
It is complete, sophisticated and ... free.
However, its use for more serious job is not very easy as it doesn't contain calibration feature and,
what's most important, it doesn't allow AC/DC V/A measurements with numeric output.
To meet our needs we developed original program. It is two channel:
AC/DC V/A meter,
with possibility to save data in the form of regular WAV file or as a CSV (comma separated variables) data file, which is easily analyzable with majority of current spreadsheet programs.
To make it a real multimeter we equipped the device with manual range switch.
The resistors used are standard ones, e. g. no special selection is needed
- calibration will ensure the final accuracy.
Resistor 1M is standard 0.25 W resistor, resistor 11M is old 0.5W one from stock.
It may be difficult nowadays, far from vacuum tubes era, to get one.
But who has stock... Anyway, it can be any in the range 5 - 15M,
serial combination of smaller resistors too...
Resistor 0.1Ohm is a small piece of some resistive wire of unknown origin
(maybe from car power controller?) from the same stock.
No special components were used.
Dual switch was constructed in small metal box with input bushings.
It is connected with LINE IN input by standard shielded stereo cable.
Switch takes very little space at the table,
PC sits near the table, so no change in the organization in the room was needed.
It was created program to use all information available.
It is quite complex one. As it can be seen from Fig. 5,
it is dual beam oscilloscope-like device combined with AC/DC multimeter,
frequency meter and VU-meter.
그림 5: SB 스코프의 주화면
■ The screen has five parts:
row above the scope screen,
row under the scope screen,
The row above the scope screen contains (from left to right):
indicator of active triggering (yellow=active),
scope channel A and B enable/disable check boxes,
triggering enable/disable and channel A or B selection.
■ 스코프의 창
폭은 스위프 범위의 선택에 따라 6us ~48 µs, 높이는 +/- 3 에 의존한다.
이래 그림의 우측을 보라.
zero level A and B,
gain A and B,
trigger level A and B.
All three controls influence only scope display.
Calibration assumes gain at maximum level.
The row below the scope screen contains (from left to right):
selector of the wave in device,
sweep speed buttons "1:1" (original speed), "1:8" (8-times slower),
calibrated or raw data display selection button,
data saving definition button - it opens small window for data definition:
■ 데이터저장 윈도우
It can be selected continual saving into regular WAV file format
or timed sample data saving into special text file (CSV).
Sampling period, capturing time and data file name can be set.
The multimeter window contains two larger and two smaller displays
- larger one displays voltage or current or raw data depending of calibration status and raw data button.
The smaller ones display frequencies in Hz.
At the right side there are range switch buttons for each channel.
They are active only if soundscope is calibrated.
AC measurement is enabled by checkboxes.
When not checked, both windows - multimeter and frequency - displays DC values.
Below those windows there are two VU-meter bars.
Right low corner is occupied by control buttons:
calibration button evoking calibration process,
start/stop data saving button,
Program was created using Borland Delphi 6.
No shareware or commercial libraries or components were used.
To control mixer it was used excellent free mixer component developed by Vit Kovalcik.
To display numeric values nice it was into Microsoft Windows installed LED display font - NI7SEG.
TTF found somewhere on the Internet.
All copyrights of the program are owned by the authors.
The noncomercial use of the program is free of any charge.
All other use must be consulted with the authors.
Program runs under Microsoft Windows 2000 and 98. It successfuly runs at AMD K6-2 333MHz/64MB RAM.
It may have problems at old machines.
Program is based on LINE IN inputs use.
It is set to use two-channel 16-bit A/D conversion at 44.1 kHz sampling frequency.
Buffer size of the input sampling is 4096 samples,
e. g. measurement frequency is 10.7 measurement per second.
Due to very low sound card sampling frequency the highest frequency
of the acceptable processed signal is around 10kHz.
Higher frequencies are processed too, but the AC accuracy drops down.
Anyway DC measurements are quite accurate, depending on calibration accuracy, of course.
The main domain of the device use should be, except use as standard multimeter,
long term DC and low frequency AC measurements.
AC amplitude measurements are achieved by software rectifying
- the value displayed is close to effective value of the AC signal.
Frequency measurement is achieved by periods count measurement.
As the measurement is kvanted by 1/44 100 s, it is also measure of the frequency display accuracy.
The smallest measurable frequency is around 20Hz.
VU-meters display maximum amplitude of the signal, regardless positive or negative,
in one sample buffer, e. g. its display refreshes with the frequency 10.7 per second.
Incorporation of VU-meter into project seems to have negligible sense.
In the future versions will be, maybe, omitted.
Scope window shows much more information.
Triggering is derived from channel A or B. There is no possibility to have triggered both channels.
Triggering level can be set. Trigger level is not dependent on display zero level.
Currently only positive levels can be set.
As the signal is sampled, e. g. not continual, the resulting triggered display is usually not fully stable.
Successful use of the program expects proper sound card setup.
It is accessible via Control panel -> Sounds and Multimedia -> Audio -> Recording setup.
For more details check Microsoft Windows help.
As personal computers can have more than one sound card, proper card selection and setup is prerequisite.
Hardware modification described in this article, provided properly set,
does not influence sound card standard features in any way.
The modified card can be still used by standard way.
Since the soundscope can be used as non calibrated, it's not its goal.
To use multimeter functions it is necessary to calibrate all ranges.
Before calibration is started, it is suggested to measure linearity and sensitivity of the LINE IN input.
It can be easily done using voltage calibration circuit described below.
To calibrate soundscope properly it is needed:
variable voltage/variable current source 1 to 15V DC/1A,
good multimeter, the best is digital one,
manual range switch properly set and connected with LINE IN input of sound card.
Calibration is four step process.
It consists from three identical steps to calibrate voltage ranges
and one step to calibrate current range.
All steps are well described by program messages.
The first of all is zero level calibration.
It requires disconnect any external voltages from inputs and shortening both with ground.
To calibrate voltage ranges one is expected to use circuit according Fig.7.
Use of regulated power source is very convenient,
anyway the calibration can be sufficiently done using batteries as power source.
In this case small lamp should be used to limit current.
Current range is calibrated with the help of circuit on Fig. 8.
It should be noted, that ground bushing is not used.
This is possible only if the power source has ground insulated from common ground.
If it is not available, battery with lamp should be used.
■ 그림 7: 전압교정
■ 그림 8: 전류교정
Calibration checks optimality of the results.
It is expected, that raw value of each range will be in interval 8000 - 24000.
If this is not achieved, program shows message. Calibration can continue,
it is not suggested, however.
Right solution is to check hardware and change preamplifier range if needed.
Calibration, if successful, ends with writing into system registry.
It means no INI or configuration file is created.
Calibration data can be found as values of the key:
On program window there is button "Raw" to disable calibration temporary.
To delete calibration permanently registry key mentioned above should be deleted using regedit.
■ 결론 기록
There are no special components used, except 11M resistor in switch.
However silicon diodes at the input should be fast ones and should have very high resistivity.
1k resistor conducting signal to them and LINE IN should be as small as possible
- its function is to be fuse.
It should burn to prevent the input from overloading.
Diodes should bear the "burning" current, of course.
For soldering at the sound card microsolder is needed.
Transformer solder must be avoided.
A good lens or glasses and a certainty in hand may be needed too...
Be careful when disconnecting input pins from ground (provided sound card has preamplifier already).
SMD chips are quite fragile!
We glued small piece of universal PCB carrying additional components close to the LINE IN input.
All joins we did with insulated thin copper wires.
■ 특징 정리
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