libbladeRF Python API missing data

[Todesstuhl]

Hallo everyone,

For some reason when I try to get RX-data from my BladeRF 2.0 Ax9 via the Python API the first ~20.000 values are garbage.
But this only happens in RX_2 mode, if I get the RX-data from ch1 & ch2 separately everything works fine.

I tried to reproduce the error with the BladeRF CLI on Windows, but these RX-data are complete.
Therefore I thing the erreor must by somewhere in the Python API.

Can someone confirm the error?

My Setup:

flowchart LR
    SG[Signal Generator] --> C["90° Coupler"]
    C --> PS[Phase Shifter]
    PS --> RX1[RX1]
     C --> RX2[RX2]

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Python-Code:

# Source: https://pysdr.org/content/bladerf.html#further-reading
# Note: This is a simplified example for demonstration purposes.

from bladerf import _bladerf
import numpy as np
import matplotlib.pyplot as plt



# Parameters
sample_rate = 40e6 #61.44e6
center_freq = 2.5e9
rx_gain = 5 # -15 to 60 dB
num_samples = int(1e6)
buffer_size = 8192 * 4 # number of bytes in our RX/TX buffer

sdr = _bladerf.BladeRF()
rx_ch1 = sdr.Channel(_bladerf.CHANNEL_RX(0)) # Zuweisung des RX1 Kanals
rx_ch2 = sdr.Channel(_bladerf.CHANNEL_RX(1)) # Zuweisung des RX2 Kanals
rx_ch1.frequency = center_freq
rx_ch1.sample_rate = sample_rate
rx_ch1.bandwidth = sample_rate/2
rx_ch2.frequency = center_freq
rx_ch2.sample_rate = sample_rate
rx_ch2.bandwidth = sample_rate/2
rx_ch1.gain_mode = _bladerf.GainMode.Manual
rx_ch1.gain = rx_gain
rx_ch2.gain_mode = _bladerf.GainMode.Manual
rx_ch2.gain = rx_gain

# Setup synchronous stream
sdr.sync_config(layout = _bladerf.ChannelLayout.RX_X2, # or RX_X2
                fmt = _bladerf.Format.SC16_Q11, # int16s
                num_buffers    = 16,
                buffer_size    = 8192,
                num_transfers  = 8,
                stream_timeout = 3500)

# Enable module
print("Starting receive")
rx_ch1.enable = True
rx_ch2.enable = True
# Create receive buffer
bytes_per_sample = 4 # don't change this, it will always use int16s
buf = bytearray(1024 * bytes_per_sample * 2) # *2 for two channels
x = np.zeros(num_samples, dtype=np.complex64) # storage for IQ samples
y = np.zeros(num_samples, dtype=np.complex64) # storage for IQ samples

# Receive loop
num_samples_read = 0
while True:
    if num_samples > 0 and num_samples_read == num_samples:
        break
    elif num_samples > 0:
        num = min(len(buf) // bytes_per_sample, 2 * (num_samples - num_samples_read))
    else:
        num = len(buf) // bytes_per_sample
    sdr.sync_rx(buf, num) # Read into buffer
    samples = np.frombuffer(buf, dtype=np.int16)
    xtmp = samples[0::4] + 1j * samples[1::4] # Convert to complex type
    ytmp = samples[2::4] + 1j * samples[3::4] # Convert to complex type
    xtmp /= 2048.0 # Scale to -1 to 1 (its using 12 bit ADC)
    ytmp /= 2048.0 # Scale to -1 to 1 (its using 12 bit ADC)
    num_cplx = num // 2

    #samples = samples[0::2] + 1j * samples[1::2] # Convert to complex type
    #samples /= 2048.0 # Scale to -1 to 1 (it is using a 12-bit ADC)
    x[num_samples_read:num_samples_read+num_cplx] = xtmp[0:num_cplx] # Store buf in samples array
    y[num_samples_read:num_samples_read+num_cplx] = ytmp[0:num_cplx] # Store buf in samples array
    num_samples_read += num_cplx
    #print("y", ytmp[0:10])

print("Stopping")
rx_ch1.enable = False
rx_ch2.enable = False

print("Max Magnitude CH1 RX_2:", np.max(np.abs(x)))
print("Max Magnitude CH2 RX_2:", np.max(np.abs(y)))
print("RMS CH1 RX_2:", np.sqrt(np.mean(np.abs(x)**2)))
print("RMS CH2 RX_2:", np.sqrt(np.mean(np.abs(y)**2)))