LabWindows/CVI
sound card pcm
Sorry, I should have previewed my post. I am using LabWindows/CVI to make a sound card oscilloscope/spectrum analyzer. I saw that others on the forum had done similar projects, so I thought someone might be familiar with the topic.
I've modified my code to fix the line-in volume to 50%. Whenever I click a "RECORD" button on my interface, the callback function performs this:
MMRESULT result; HWAVEIN waveHandle; WAVEFORMATEX waveFormat; short int waveData[220500]; WAVEHDR waveHeader; HMIXER mixerHandle; MIXERLINE mixerLine; MIXERLINECONTROLS mixerLineControls; MIXERCONTROL mixerControl; MIXERCONTROLDETAILS mixerDetails; MIXERCONTROLDETAILS_UNSIGNED lineInVolume; waveFormat.wFormatTag = WAVE_FORMAT_PCM; // standard uncompressed PCM waveFormat.nChannels = 1; // mono waveFormat.nSamplesPerSec = 44100; // 44.1 kHz sample rate waveFormat.wBitsPerSample = 16; // 16 bits per sample waveFormat.nBlockAlign = 2; // 2 bytes per sample waveFormat.nAvgBytesPerSec = 88200; // 44.1 kHz * 2 bytes per sample waveFormat.cbSize = 0; // no extra data appended for PCM // open waveform audio input, no ACM result = waveInOpen( &waveHandle, WAVE_MAPPER, &waveFormat, 0, 0, CALLBACK_NULL | WAVE_FORMAT_DIRECT ); // open mixer associated with waveform input result = mixerOpen( &mixerHandle, (UINT)waveHandle, 0, 0, MIXER_OBJECTF_HWAVEIN ); mixerLine.cbStruct = sizeof(MIXERLINE); // initialize size mixerLine.dwComponentType = MIXERLINE_COMPONENTTYPE_SRC_LINE; // look for line-in source mixer line // get line info about the line-in source mixer line result = mixerGetLineInfo( (HMIXEROBJ)mixerHandle, &mixerLine, MIXER_GETLINEINFOF_COMPONENTTYPE ); mixerLineControls.cbStruct = sizeof(MIXERLINECONTROLS); // initialize size mixerLineControls.dwLineID = mixerLine.dwLineID; // line-in source mixer line ID mixerLineControls.dwControlType = MIXERCONTROL_CONTROLTYPE_VOLUME; // look for volume control of line-in mixerLineControls.cbmxctrl = sizeof(MIXERCONTROL); // initialize size mixerLineControls.cControls = 1; // one control (volume) mixerLineControls.pamxctrl = &mixerControl; // assign control storage structure // get the volume control of the line-in source mixer line result = mixerGetLineControls( (HMIXEROBJ)mixerHandle, &mixerLineControls, MIXER_GETLINECONTROLSF_ONEBYTYPE ); mixerDetails.cbStruct = sizeof(MIXERCONTROLDETAILS); // initialize size mixerDetails.dwControlID = mixerControl.dwControlID; // line-in volume control ID mixerDetails.cChannels = 1; // one channel (mono) mixerDetails.hwndOwner = 0; // no window handle mixerDetails.cMultipleItems = 0; // no multiple items mixerDetails.paDetails = &lineInVolume; // assign volume storage structure mixerDetails.cbDetails = sizeof(MIXERCONTROLDETAILS_UNSIGNED); // initialize size lineInVolume.dwValue = 0x7FFF; // set volume to this value (50%) // set volume control of the line-in source mixer line result = mixerSetControlDetails( (HMIXEROBJ)mixerHandle, &mixerDetails, MIXER_GETCONTROLDETAILSF_VALUE ); waveHeader.dwBufferLength = 5 * 88200; // 5 seconds * 88200 bytes per second waveHeader.dwFlags = 0; // initialize flags to 0 waveHeader.lpData = (LPSTR)waveData; // assign waveform data storage structure // prepare header result = waveInPrepareHeader( waveHandle, &waveHeader, sizeof(waveHeader) ); // add buffer result = waveInAddBuffer( waveHandle, &waveHeader, sizeof(waveHeader) ); // start recording result = waveInStart( waveHandle ); // wait until recording is finished while ( waveInUnprepareHeader ( waveHandle, &waveHeader, sizeof(waveHeader) ) == WAVERR_STILLPLAYING ); // close waveform input and mixer devices result = waveInClose( waveHandle ); result = mixerClose( mixerHandle ); DeleteGraphPlot (panelHandle, PANEL_CHANNEL_LEFT, -1, VAL_IMMEDIATE_DRAW); PlotWaveform (panelHandle, PANEL_CHANNEL_LEFT, waveData, 220500, VAL_SHORT_INTEGER, 1.0, 0.0, 0.0, 1.0/44100, VAL_THIN_LINE, VAL_NO_POINT, VAL_SOLID, 1, VAL_RED);
However, the data plotted to the graph window is not at a static volume level. In the next three samples, I am recording 5 seconds with no input to line-in, so I should record low-level white noise at constant amplitude. But I get the following:
And I don't have a clue as to what is happening. It's as though there's some sort of AGC or compression on, but I have no idea how to control it. Does anybody have any ideas?
