added bandwidth estimation details in the README.md

README.md

@@ -83,6 +83,11 @@ During download there might be warning messages from "libmpg123". These warnings
 
 By default, the script will download audio files into the workspace directory under *{workspace_dir}/audio_22khz*. The download will ignore HTTP errors and store information for any failed downloads into *{workspace_dir}/errors_22khz.json*. A new manifest will be created at *{worksapce_dir}/manifest_filtered_22khz.json* with utterances from failed audiobooks removed. You can override the default behavior by modifying the [config.yaml file](https://github.com/NVIDIA/NeMo-speech-data-processor/blob/main/dataset_configs/english/hifitts2/config_22khz.yaml) in your local SDP repository.
 
+We apply the bandwidth estimation approach from the [`estimate_bandwidth.py`](https://github.com/NVIDIA/NeMo-speech-data-processor/blob/main/sdp/processors/nemo/estimate_bandwidth.py#L65) in [Speech Data Processor (SDP) Toolkit](https://github.com/NVIDIA/NeMo-speech-data-processor) to the first 30 seconds of each audiobook. The bandwidth fmax is estimated by using the mean of the power spectrum to find the highest frequency that has at least -50 dB level
+relative to the peak value of the spectrum, namely,
+$$f_{\text{max}} = \max\left\{f \in [0, f_{\text{Nyquist}}] \, \bigg|\, 10 \log_{10} \left(\frac{P(f)}{P_{\text{peak}}}\right) \geq -50\, \text{dB}\right\}$$
+where `P(f)` is the power spectral density and `P_peak` the maximum spectral power.
+
 If you want to retry the download for failed audiobooks, rerun the script with the output *errors_22khz.json* file. 
 
 ```bash