How does sound quality change when listening from an iPhone via Bluetooth? Waveform comparison

There are several codecs for Bluetooth audio transmission. SBC, which was common in the past, has recently become mainstream, so AAC and aptX have become mainstream, so compared to previous Bluetooth headphones, the sound quality has improved and the latency has been reduced. In addition, aptX HD, which goes beyond aptX, has appeared, and in the case of Sony, LDAC, a unique codec with higher sound quality, is installed in each product. is a well-known fact.

However, I don't really know how much it differs from wired headphones and earphones, and there seems to be little information. Therefore, I decided to conduct an experiment to clarify the difference.

Here's the idea. First, play the WAV file and fly it over Bluetooth. Accurately capture incoming Bluetooth audio data and compare it with the original WAV file. At this time, it is a calculation that the difference becomes clear if you take the difference or compare it by frequency analysis.

The problem is how to capture Bluetooth audio data accurately. Receiving this with Bluetooth earphones or Bluetooth headphones will not work, so I want to capture it using a Bluetooth receiver. Some Bluetooth receivers have not only an analog output but also a S/PDIF digital output, so I searched for an accurate capture using this. I searched for a Bluetooth 4.x compatible receiver that supports AAC and aptX, but I couldn't find a suitable product. Instead, I found Elecom's "LBT-AVWAR700" as a receiver that supports Bluetooth 3.0, AAC and aptX, and has S/PDIF output, although it is a slightly old product.

Bluetooth 3.0 consumes more power than Bluetooth 4.x, but the file transfer speed is even faster, and the purpose here is to improve the sound quality with AAC and aptX. Since it was something to check, it seemed like there would be no problem as experimental equipment, so I decided to purchase it online and try it out.

The next day, I received the product, so when I connected it to my iPhone X, I was able to confirm that it worked without any problems. When I tried to get this optical digital output into my PC, I realized that there was no connectable audio interface. S/PDIF, which used to be standard equipment in many audio interfaces, has recently disappeared, and the audio interfaces I usually use do not support it.

I couldn't help it, so I pulled out some old audio interfaces from the warehouse and tried them out. However, in the case of cheap audio interfaces, sampling rate converters are included and accurate capture is not possible. So I tried it with Roland's UA-101, which I used to use a lot in the past, and found that it was able to capture accurately with a solid lock at 44.1kHz, so I used it.