oscilloscope using audio line in on laptop

Discussion in 'General Electronics Chat' started by TheSoundshark, Nov 25, 2010.

  1. TheSoundshark

    Thread Starter New Member

    Nov 24, 2010
    I was having a think late last night about saving my self some money and getting a second hand oscilloscope but then an idea hit me what if i reduced the voltage all way down between 0.4 - 1V, that would be at line level accepted by standard audio equipment. Now i could then connect this to a notebook running free audio software to record the wave or see in real time. i understand its not going to be as simple as just stick a transformer in there but i do think it may be a viable option.

    I would really like your thoughts on this and any ideas you may have to implement it


  2. tom66

    Senior Member

    May 9, 2009
    This is not a new idea. What you are talking about is a sound card oscilloscope.

    However there are several limitations you should be immediately aware of:
    • Maximum bandwidth is audio - or about 20-25 kHz.
    • Sampling rate is maximum 44,100 on most laptops, about enough to capture a 4.41 kHz sine wave with good detail.
    • The input is AC coupled which means you can't measure DC voltages.
    • No offset control.
    • Input 1Vp-p max.
    • If you damage your sound card on your laptop you will often damage other things as it is often integrated onto one "media" chip which handles graphics, audio and external interfaces like USB.
    These specifications contrast to a real oscilloscope.
    • At least 10 MHz bandwidth. Most available in 100 MHz. (Which I recommend: 10 MHz really isn't enough.)
    • If analog, no sampling rate; if digital, usually at least 10 MS/s, newer ones do 1 GS/s.
    • At least DC and AC coupling, some offer GND coupling as well.
    • Ability to offset waveform on the display.
    • At least 100V input protection; often up to 400V.
    • Difficult to damage: if you do damage it though, you only need to replace the scope, not the entire laptop.
  3. TheSoundshark

    Thread Starter New Member

    Nov 24, 2010
    i was thinking about using a pc for testing an old win 98 system but with a higher spec audio card i have a few spare from old studio work, 64bit should give me some more head room. i see what your saying about DC also but i thought maybe in the proto type i could just use a transistor and atleast then i could add squar waves to the list well atleast the frq , if not maybe conect it directly using a resistor know what % i rudced the supply signal by i could then adjust the volume on the software to get a better reading on its voltage. Im pretty new to a lot of these concepts but i always think atleast by trying im learning :)
  4. bertus


    Apr 5, 2008
  5. tom66

    Senior Member

    May 9, 2009
    A 64-bit sound card???

    Most digital oscilloscopes are 8 bit. Some are 12 and 14 bit. This typically provides enough bandwidth. 64 bit would probably be able to discern a local radio station from the noise.

    You can't add DC: the sound card blocks this. So you need to externally measure DC, then add this to the AC signal, or have some external device measuring the DC and sending it to a computer.
  6. timrobbins

    Active Member

    Aug 29, 2009
    I use TrueRTA - with a few homemade dividers and limiters it is great for audio testing - but you do need to cross-check your results for validity. Mains loops can be noticeable for low noise work, unless you run a laptop.