rtlsdr ppm correction, how

[janvgils]

How can we correct the ppm value with soapyrtlsdr?
The current SoapySDRUtil --probe="driver=rtlsdr" isn't showing a corr=value option anymore.

`SoapySDRUtil --probe="driver=rtlsdr"
######################################################

Soapy SDR -- the SDR abstraction library

######################################################

Probe device driver=rtlsdr
Found Rafael Micro R820T tuner
Found Rafael Micro R820T tuner

---

-- Device identification

driver=RTLSDR
hardware=R820T
origin=https://github.com/pothosware/SoapyRTLSDR
rtl=0

---

-- Peripheral summary

Channels: 1 Rx, 0 Tx
Timestamps: NO
Other Settings:
* Direct Sampling - RTL-SDR Direct Sampling Mode
[key=direct_samp, default=0, type=string, options=(0, 1, 2)]
* Offset Tune - RTL-SDR Offset Tuning Mode
[key=offset_tune, default=false, type=bool]
* I/Q Swap - RTL-SDR I/Q Swap Mode
[key=iq_swap, default=false, type=bool]
* Digital AGC - RTL-SDR digital AGC Mode
[key=digital_agc, default=false, type=bool]

---

-- RX Channel 0

Full-duplex: YES
Supports AGC: YES
Stream formats: CS8, CS16, CF32
Native format: CS8 [full-scale=128]
Stream args:
* Buffer Size - Number of bytes per buffer, multiples of 512 only.
[key=bufflen, units=bytes, default=262144, type=int]
* Ring buffers - Number of buffers in the ring.
[key=buffers, units=buffers, default=15, type=int]
* Async buffers - Number of async usb buffers (advanced).
[key=asyncBuffs, units=buffers, default=0, type=int]
Antennas: RX
Full gain range: [0, 49.6] dB
TUNER gain range: [0, 49.6] dB
Full freq range: [23.999, 1764] MHz
RF freq range: [24, 1764] MHz
CORR freq range: [-0.001, 0.001] MHz
Sample rates: 0.25, 1.024, 1.536, 1.792, 1.92, 2.048, 2.16, 2.56, 2.88, 3.2 MSps`

[kerel-fs]

To answer a part of the question our-self: There are two possible ways to set the frequency correction via the provided SoapySDR APIs:

1. via Frequency API by SoapyRTLSDR::setFrequency with name="CORR":

   const double corr_frequency = 49;  # in ppm
   std::vector<SoapySDR::Kwargs> d_tune_args;
   d_device->setFrequency(SOAPY_SDR_RX, channel, "CORR", corr_frequency, d_tune_args);
   

2. via Frontend corrections API by SoapyRTLSDR::setFrequencyCorrection

   const double corr_frequency = 49;  # in ppm
   d_device->setFrequencyCorrection(SOAPY_SDR_RX, channel, corr_frequency);
   

(Note that the underlying rtlsdr_set_freq_correction method supports integer values only (include/rtl-sdr.h#L155-L162) while both Soapy APIs accept doubles and type-casts them.)

The only question remaining:
Which of those two possibilities is preferred / do you plan to deprecate one of the two APIs in the future?

[cjcliffe]

Looks like setFrequencyCorrection was added two years after the initial method of using "CORR" as a workaround was implemented.

Use setFrequencyCorrection and consider calling setFrequency with "CORR" to be deprecated -- I'll be updating my own application to use the new function as well so that it will work with all supported devices.