Audio engineer David Sum is a man who knows the nuts and bolts of all those notes and chords He has worked with legendary singers like the late Leslie Cheung Kwok-wing and Anita Mui Yim-fong, and has composed - technically, at the beat-by-beat level - thousands of songs that have made their way onto released albums. In his studio in Wan Chai, David Sum, probably the most experienced audio engineer in Hong Kong, is always ready to record and mix any type of music. 'After I graduated from school, I worked as a salesman at Tom Lee Music, where I got to know some musicians. It wasn't long before I was offered a job in a studio and I have been in this field for 25 years now,' recalls Sum. 'People may not be aware that there are many stages in the production of a song. Besides the creation of music and lyrics, technical expertise is involved and that's where audio engineers, or 'mixers', come in. 'Each song comprises lots of tracks. While a simple song may only have 40 tracks, an average musical piece involves 100 or more. A track is a pure sound element. It can be a human vocal, a harmony or instruments like drum or guitar. For example, there are as many as 16 different kinds of tracks for drums only. The decision on what kinds of instruments are used belongs to the producer, but we sometimes give advice. 'We work out all the technical details, such as the volume and intensity of each track. We take care of the quality of the sound - what feeling it will give to the audience. While artists sing to express the song's musical element, sound mixing aims to enhance the artist's performance. 'If the producer or the musician has already developed a picture of a particular song in their mind, our job is to decode the picture into something solid for the audience.' An audio engineer has the 'the second most boring job on Earth,' jokes Sum, 'only next to audio engineer assistant'. You may have to listen to songs again and again and again. 'You have to be very interested in music to do this job. You could hear a four-minute song some 200 times if you also do the recording. But each time you listen to a song, you get different feelings and elements from it. And these trigger ideas for ways to mix the song.' It is exactly these feelings that allow room for creativity and variety. Despite having to follow the producer's direction, veteran audio engineers always have their own style. 'You add your own feeling and use your own judgment when you mix a song. If you mix a song twice, you can never mix it exactly the same way both times. Every mix of a song is a stand-alone product of the feeling at that exact moment. 'Once I took around five hours to make a first draft of a song for one producer. After listening to it, he said 'It is completely not what I wanted.' 'I thought I'd messed up and wasted my time, until he said 'But I like it.' Not every producer can accept different opinions. It is the recognition from artists and producers that give me my job satisfaction.'