Audio reproduction of 78 rpm records by analyzing the pictures of the record surface with a digital camera

78 rpm records are valuable historical materials, but conventional playback methods using a stylus tend to damage them. Moreover, some records are difficult to play due to scratches, dust, or cracks. Therefore, non-contact methods for playing records are desired. There has been some research on non-contact playback, but most of it used large devices. In this study, we used an ordinary digital camera to reconstruct the record sound. We photographed the record in eight segments, each covering one-eighth of a rotation. For each image, we performed a transformation from polar to Cartesian coordinates. The grooves of a record appear as horizontal stripes representing valleys and peaks in pixel values. The narrow peaks in the image corresponding to the bottom of the grooves are sharp enough to extract the groove shape accurately. Since the shape of the grooves correspond to the sound waveform, the coordinates of the peaks can be regarded as the discretized audio data. After combining the coordinates and calculating the sampling frequency from the rotation speed, we reconstructed the sound. The resulting audio was noisier than the audio from stylus, but sufficient to identify the corresponding parts.