The sample rate of my sound card changes slightly. Counteract? - vb.net

Question

I read this thread because I also wanted to find out if the sample rate of my sound card is always the same or if the value fluctuates. I downloaded the Soundcheck programme recommended in the thread. According to the Windows settings, my sound card is set as follows:
Sample rate: 48000 Hz
Channels: 2
Resolution: 24 bits.
A long time ago I wrote an application that does a Fourier transform and displays the magnitudes of the individual frequencies as a bar (chart).
I have recorded a video for you because it is difficult for me to explain the problem. As you can see, according to Soundcheck, the bytes per second are changing all the time. I have also seen larger value deviations (up to ±1 %). I claim that this problem is the reason why the bars in the Fourier transformation programme jump up and down a bit, especially those that should not be visible at all.
Please note that due to the video editing programme it seems as if the played 2000 Hz test tone trills, but it does not. It is always the same volume.
Is there anything that can be done to get the true sample rate of the sound card, or is the code for getting the audio bytes not good enough?
I will delete the video in a few days.
This is the VB.NET source code to get the audio data (minimal example). You must download NAudio from Visual Studio's own NuGet package manager.
Imports Microsoft.VisualBasic.ControlChars
Public NotInheritable Class FormMain
Private continue_ As Boolean = True
''' <summary>
''' sample rate of the soundcard in Hertz
''' </summary>
Private Samplerate As Integer = 48000
''' <summary>
''' 1024
''' </summary>
Private ReadOnly BufferSize As Integer = CInt(Math.Pow(2, 10)) ' 1024
Private Shared wi As NAudio.Wave.WaveIn = Nothing
Private bwp As NAudio.Wave.BufferedWaveProvider = Nothing
''' <summary>
''' 21.333 ms
''' </summary>
Private usedMilliseconds As Double
Private cnt As Integer = 0
''' <summary>
''' to hold the latest soundcard data that we will graph. <para></para>
''' Size of this array becomes BufferSize ÷ 2, i.e., 512.
''' </summary>
Private DataArray As Double()
Private Sub AudioDataAvailable(ByVal sender As Object, e As NAudio.Wave.WaveInEventArgs)
bwp.AddSamples(e.Buffer, 0, e.BytesRecorded)
End Sub
Private Sub Start_Listening()
wi = New NAudio.Wave.WaveIn()
wi.DeviceNumber = 1
wi.WaveFormat = New NAudio.Wave.WaveFormat(Samplerate, 16, 1)
wi.BufferMilliseconds = CInt(CDbl(BufferSize) / CDbl(Samplerate) * 1000.0)
usedMilliseconds = CDbl(BufferSize) / CDbl(Samplerate) * 1000.0
AddHandler wi.DataAvailable, AddressOf AudioDataAvailable
bwp = New NAudio.Wave.BufferedWaveProvider(wi.WaveFormat)
bwp.BufferLength = BufferSize * 2
bwp.DiscardOnBufferOverflow = True
Try
wi.StartRecording()
Catch ex As NAudio.MmException
MessageBox.Show($"Could not record from audio device!{NewLine}{NewLine}{ex.Message}",
"",
MessageBoxButtons.OK,
MessageBoxIcon.Error)
End Try
End Sub
Public Sub GetLatestData()
Dim audioBytes As Byte() = New Byte(BufferSize - 1) {}
bwp.Read(audioBytes, 0, BufferSize)
If audioBytes.Length = 0 Then Return
'If audioBytes(BufferSize - 2) = 0 Then Return
Dim BYTES_PER_POINT As Integer = 2
Dim PointCount As Integer = audioBytes.Length \ BYTES_PER_POINT
DataArray = (New Double(PointCount - 1) {})
For i As Integer = 0 To PointCount - 1 Step 2
DataArray(i) = CDbl(BitConverter.ToInt16(audioBytes, i * BYTES_PER_POINT))
Next
End Sub
Private Async Sub ButtonStart_Click(sender As Object, e As EventArgs) Handles ButtonStart.Click
Start_Listening()
continue_ = True
While continue_
Await Task.Run(Sub() FT())
Await Task.Run(Sub() Plot())
End While
End Sub
Private Sub ButtonStop_Click(sender As Object, e As EventArgs) Handles ButtonStop.Click
If wi IsNot Nothing Then
wi.StopRecording()
RemoveHandler wi.DataAvailable, AddressOf AudioDataAvailable
wi.Dispose()
End If
continue_ = False
PictureBox1.Image = Nothing
GC.Collect()
End Sub
Private Sub Plot()
' plot the bars
End Sub
Private Sub FT()
GetLatestData()
If DataArray Is Nothing Then
Return
End If
' do a fourier transform here
End Sub
Private Sub NumericUpDown_Samplerate_ValueChanged(sender As Object, e As EventArgs) Handles NumericUpDown_Samplerate.ValueChanged
Me.Samplerate = CInt(NumericUpDown_Samplerate.Value)
End Sub
End Class
Addition for future readers
May 2, 2022, 11:19 a.m.,
Thank you, Hans, for this interesting article. No sarcasm. The article describes exactly the problems I am experiencing and states that this is normal and because the measurement data is finite instead of infinite.
My problems are in fact:
1.) The rectangles in the chart of wrong, neighbouring frequencies also rise.
2.) There is a wave effect, i.e., the individual bars rise and fall from right to left like a wave. “…an FFT of these data produces spectral values “riding” on a curve, not a single point at a single frequency as you might expect.”
Figure 3b best illustrates the situation: a pure 17.5 Hz sine wave shows up in the graph making the neighbouring points at 16 Hz and 18 Hz rise (instead of being peaked at 17.5 Hz).
So I assume (as of now) that the approach to get the sound card data is sufficient and there are simply some errors which are documented in scientific papers.