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[Bug]: Time-stretched proxies are written at the source's bit depth

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
68/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
cpp
Domain
audio-video-rtc

Research direction

The bit-depth choice is in the proxy writer in modules/tracktion_engine/model/clips/tracktion_AudioClipBase.cpp, around line 158, where std::max (16, sourceInfo.bitsPerSample) is passed to AudioFileWriter. Read how proxyInfo is set to tell time-stretched proxies from normal ones. Done means time-stretched proxies are written as 32-bit float while normal-speed proxies keep the source bit depth. Check this with a stretched and an unstretched clip, since the patch in the issue is unverified.

Written by the indexing model from the issue text.

Description

Summary. ProxyGeneratorJob writes a time-stretched proxy with std::max (16, sourceInfo.bitsPerSample) bits, i.e. 16 or 24-bit integer. The stretcher's output is new audio: it gets requantised and clipped if it exceeds 0 dBFS.

Proposal (patch below): write time-stretched proxies as 32-bit float; normal-speed proxies keep the source's bit depth.

Environment: develop @07e2d674 with its JUCE 8.0.13, macOS arm64, 48 kHz, TRACKTION_ENABLE_TIMESTRETCH_SIGNALSMITH=1. The code in question is unchanged on develop @38d5a275.

Patch: 001-proxy-32bit-float.patch 
diff --git a/modules/tracktion_engine/model/clips/tracktion_AudioClipBase.cpp b/modules/tracktion_engine/model/clips/tracktion_AudioClipBase.cpp
index b3d2852..6e8964e 100644
--- a/modules/tracktion_engine/model/clips/tracktion_AudioClipBase.cpp
+++ b/modules/tracktion_engine/model/clips/tracktion_AudioClipBase.cpp
@@ -158,9 +158,13 @@ private:
         if (! wavFormat->isChannelLayoutSupported (sourceInfo.channelLayout))
             sourceInfo.channelLayout = juce::AudioChannelSet::discreteChannels (sourceInfo.channelLayout.size());
 
+        // A time-stretched proxy is new audio, not a copy of the source: write it as 32-bit float
+        // so it isn't requantised to the source's bit depth or clipped above 0 dBFS
+        const int bitsPerSample = proxyInfo != nullptr ? 32 : std::max (16, sourceInfo.bitsPerSample);
+
         AudioFileWriter writer (tempFile, wavFormat,
                                 sourceInfo.numChannels, sourceInfo.sampleRate,
-                                std::max (16, sourceInfo.bitsPerSample),
+                                bitsPerSample,
                                 sourceInfo.metadata, 0,
                                 sourceInfo.channelLayout);
Dominant language
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