Why is the sound quality of Bluetooth headphones so poor?
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If anyone knows a little about audio, they will know that the sound quality of Bluetooth headphones is not as good as that of wired headphones. Some people attribute the sound quality problem to the transmission speed of Bluetooth, and believe that the upgrade of Bluetooth standards will inevitably lead to improved sound quality. But is this really the case? What are the reasons that limit the sound quality of Bluetooth audio devices? Lossy compression during Bluetooth transmission The fundamental reason for the poor sound quality of Bluetooth audio devices is very simple, because lossy coding is used to compress the audio signal during the transmission process. In order to achieve a larger compression ratio and a smaller data volume, lossy coding will discard some of the original information in the audio signal during compression. The compressed information is insufficient, resulting in distortion during the decompression and restoration process. This is what the distortion of lossy audio is like I think many people know that over-compressed images will produce obvious noise or color blocks, and these two phenomena are the distortion caused by lossy compression. The same is true for lossy compression of audio, which will also introduce distortion during restoration, but it is not very intuitive. Many people find it difficult to distinguish between lossy and lossless music, so it is normal even if they do not feel the loss of sound quality caused by Bluetooth transmission. If you want to distinguish the obvious difference between lossy and lossless, I suggest compressing a lossless music into a low-bitrate MP3 (preferably less than 96kbps). After comparison, it can be clearly found that the lossy mp3 version will lose a lot of details and high frequencies. At the same time, the density of the sound will also decrease, and you will feel that lossy audio has a very loose listening experience. However, as the bit rate (file size) of lossy audio increases, the difference will become smaller and smaller, or even difficult to distinguish, which is far less intuitive than images and videos. Why use lossy transmission? In the early days, because the Bluetooth bandwidth was limited, lossy compression had to be used to reduce the audio bit rate before transmission. Today, Bluetooth has gone through several generations of development, and the Bluetooth 4.0 limit transmission can reach 24Mbps. Looking at it now, the bandwidth requirement for lossless audio is not harsh. CD-specification lossless audio only requires 1.4Mbps bandwidth, which is far lower than the limit speed, but why is it still impossible to achieve lossless transmission? The sound quality of Bluetooth transmission may be limited by two aspects. First, it is because of the protocol. Bluetooth audio transmission mainly relies on the A2DP (Advanced Audio Distribution Profile) protocol. A2DP specifies the conditions that must be met when audio is transmitted via Bluetooth. If the A2DP protocol is not used for transmission, the receiver will not be able to recognize the audio data sent by the sender. This protocol was established in the early days of the Bluetooth standard. It did not take into account the current situation and limited the amount of data during transmission, so that audio data could only reach about 328kbps. Therefore, even if the new Bluetooth standard can bring higher transmission speeds, these protocols have not been updated and will still be limited to the speeds they limit when used. Secondly, Bluetooth is also based on the 2.4GHz frequency band, and WiFi, wireless keyboards and mice all use this frequency band. When the data is large, some interference will occur, resulting in intermittent sound, especially audio devices will have a significant impact on the user experience. The sound quality is actually gradually improving. Even if the amount of data during Bluetooth audio transmission is limited by the protocol, the sound quality can be improved by improving the compression quality of the transmission encoding. The very popular aptX encoding is achieved in this way. Simply put, it can bring better compression effects under the same bit rate, so even if it is limited by the protocol, it can still improve the sound quality. Although aptX improves the sound quality, it is not lossless, and there is still a gap with the sound quality of lossless audio. At present, the only technology that can achieve Bluetooth lossless audio transmission should be Sony's LDAC technology, which is actually a different approach. In order to achieve this, Sony adopted its own unique method. Therefore, LDAC is not universal and can only be used on Sony's own audio equipment or mobile phones and Walkman. Of course, it is not ruled out that it will be authorized to other manufacturers in the future. In fact, Sony's LDAC technology is not perfect. LDAC has three compression modes. If the highest quality (bit rate) is used for transmission, it is easy to be interfered and cause intermittent sound. Sony also gave a prompt in the corresponding settings interface, suggesting that we turn off WiFi, which is also based on 2.4GHz. So from this point, we can know why Bluetooth audio has not been fully popularized and lossless transmission has not yet been achieved, because it is not stable to do so. It will affect the user experience, and this is definitely a more difficult problem to solve, otherwise Sony, which is full of black technology, would have solved it long ago. The impact of built-in DAC and amplifier in Bluetooth audio devices cannot be ignored . In addition to the problems caused during transmission, since Bluetooth audio devices receive digital signals, DAC and amplifiers need to be integrated internally. For beauty and portability, Bluetooth speakers and headphones must also consider volume. Limited by the limited space, the circuit design of DAC and amplifier will be affected, which will limit the sound quality and fail to bring the same performance as independent DAC and amplifier. Moreover, compared with passive speakers or wired headphones, Bluetooth audio devices cannot be matched at will, which will affect the playability. Even if there is a sufficient budget, it is impossible to equip it with a better DAC and amplifier. What will happen in the future? Just recently, the Bluetooth 5.0 standard has been released. It is said that it will improve the transmission distance and speed, rather than focusing on power consumption control like the previous two generations. It seems that lossless Bluetooth lossless audio transmission is promising, but this is only a big premise, because in addition to the Bluetooth standard, related protocols need to be followed up. At the same time, corresponding upgrades must be made in compression coding, supporting devices, and built-in DAC. From the fact that the high-quality aptX coding is gradually being used by more manufacturers, it can be seen that it is only a matter of time before Bluetooth audio devices can fully realize lossless transmission.As a winner of Toutiao's Qingyun Plan and Baijiahao's Bai+ Plan, the 2019 Baidu Digital Author of the Year, the Baijiahao's Most Popular Author in the Technology Field, the 2019 Sogou Technology and Culture Author, and the 2021 Baijiahao Quarterly Influential Creator, he has won many awards, including the 2013 Sohu Best Industry Media Person, the 2015 China New Media Entrepreneurship Competition Beijing Third Place, the 2015 Guangmang Experience Award, the 2015 China New Media Entrepreneurship Competition Finals Third Place, and the 2018 Baidu Dynamic Annual Powerful Celebrity. |
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