How can I re-encode a recording from an audio stream in real time?

Can I capture an audio stream that uses Variable Bit Rate (VBR) encoding and simultaneously re-encode it with Constant Bit Rate (CBR) encoding as it records?

Currently, I’m downloading an AAC format audio stream with VBR encoding using cURL. The issue is that the file’s duration, determined by its byte length, varies across different players due to the VBR encoding. This variation hinders precise seeking and slicing in the audio. To resolve this, I’m considering re-encoding the stream with a constant bit rate for more accurate seeking.

However, the stream is several hours long, and re-encoding it post-recording is too time-consuming and resource-intensive.

Is there a workaround for this? Maybe a way to set cURL to record with a constant bit rate, or another method to achieve CBR encoding on the fly?

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Yes, re-encoding a VBR audio stream to CBR in real-time is feasible, but it takes more than just cURL to do. Although cURL is an excellent tool for obtaining files, audio encoding is not one of its built-in features. For real-time audio encoding, you would want an additional device or piece of software.
Using a streaming media server or a command-line program like FFmpeg, which can record and re-encode the stream at the same time, is one popular method.

I see. Can you explain how FFmpeg can be used for this purpose? I’m concerned about the processing power and time it might take, as the stream is several hours long.

FFmpeg is efficient in handling such tasks and is widely used for real-time audio and video processing. When you use FFmpeg, it can capture the incoming stream and re-encode it to CBR on the fly. The command for this would involve specifying the input as the URL of the stream and setting the desired bitrate for the CBR encoding. FFmpeg is optimized for performance, so it should be manageable on your system, even for a long stream.

That sounds promising. But will this approach handle the issues with seeking and slicing accurately in the CBR-encoded output?

Yes, it should. One of the advantages of CBR encoding is that it makes the timing and duration of the audio stream more predictable and consistent. This regularity should resolve the issues you are experiencing with seeking and slicing in a VBR-encoded stream.
The key is to ensure that the CBR bitrate you choose is suitable for maintaining the audio quality you need.

To ensure the best balance between quality and file size, you will have a few parameters to tweak. The most important one is the bitrate. A higher bitrate will result in better audio quality but a larger file size.
Conversely, a lower bitrate saves space but might reduce quality. For AAC, bitrates around 128 kbps to 256 kbps are common for good quality. You can start with a mid-range bitrate and adjust based on the quality of the output. The choice of bitrate might also depend on the nature of the audio content you’re streaming.