Digital Audio Track Authoring

21 May

In year 1970 I had an opportunity to handle first time an Akai 8 inch reel tape stereo recorder, bought by my elder brother. He allowed me to learn and experiment audio recordings. I was well versed with Mono and Stereo audio concept. I learnt various technical terms used to measure and express audio quality during recording and then play back audio quality, the devices responsible to maintain the originality of an audio source. Those days 8 inch standard speaker were my headphones to feel the audio quality from very close distance.

I had done many audio recordings on reel and cassette tape media as personal favors to friends and family members. From 1985 onwards there were chances to do commercial audio recordings of musical groups. This was changed  when I bought Apple Mac G3 in 1996. Then I came to know the advantage of digital audio, quality, devices, editing and then final output media.

I learnt the digital audio technology and the techniques by my own method of experimenting and troubleshooting, for that I  collected maximum possible information through available media. I do not weigh the technologies and devices. I give importance to capture and preserve every possible moments by using affordable media at affordable price. So for I tried to digest certain basics of sound and the technical terms.

The compression and decompression of air pressure caused by sound variations are sensed by our ears and we termed it as Sound, a laughter, clapping or falling of kitchen items. If the sound can not give expected information then it is termed as Noise, unknown spoken language can be noise. But sound having certain repetitive information in certain time duration is termed as Audio, voice of a person or note of an instrument. When this audio information is used in a man made electrical / electronic device then it is termed as Audio Signal. Every audio signal source is considered as a track in a track recording system. There are two basic recoding system Analog and Digital.Microphone audio signal is analog, a continuous stream, recorded on conventional tape recorders having four track in one direction. The digital recording is sampled with high frequency and the signal level with bits of information. I find digital track recording and editing is much more simple, affordable and non destructive, that is during editing the original audio remain intact, in comparison to analog recording / editing system.

One cycle is a change from high pressure to low pressure and back to high pressure. The number of cycles per second are called as frequency and the measuring unit is Hertz, abbreviated “Hz.”

The human ear can sense a pressure change as small as 0.0002 micro bars (1 micro bar = 1/1,000,000 atmospheric pressure). The threshold of pain is about 200 micro bars. 0.0002 micro bar is 0 dB SPL, which is the threshold of hearing and 120 dB SPL is the threshold of pain. 1dB is about the smallest change in SPL that can be heard. A 10dB SPL increase is perceived to be twice as loud than 1db SPL. Human ear sense audio in logarithmic scale that is why unit Decibel or db is used.

Dynamic Microphone, has a  diaphragm, Voice coil, magnet assembly. Sound waves strike the thin plastic diaphragm which vibrates an attached voice coil kept free to move on the magnet. This movement of the coil generates electrical signal in that coil corresponding to the sound picked up by the dynamic microphone. Exactly opposite function of speaker is achieved by same basic parts  of a mic in larger scale and strong parts. In a speaker strong audio signal moves the coil on the magnet and the attached hard paper cone converts that audio as sound waves in free air.

Condenser or capacitive microphone is based on an electrically charged diaphragm(2) and back plate assembly(3) which forms a sound-sensitive capacitor. The battery(4) gives electrical charge to the back plate Sound waves vibrate metal-coated-plastic diaphragm. The motion of the diaphragm causes change in electrical potential which is the replica of the original sound waves. The audio signal is between Point (5) and other terminal of battery. With little difference of stronger parts the condenser mic can be used as speaker in small mobile devices.

There is Magnetic pickup used on electric metal string guitar, the vibration caused by strings is converted as audio signal.

These are various models of Microphones each has special functionality and audio quality for the serious users, but for a layman all are same, converting sound to audio signal. To select a right kind of mic one should know the specifications, terminology, such as sound Pick up pattern, Frequency response curve or graph, Sensitivity to sound pressure. There are three basic sound Pick up patterns of mics, Omnidirectional, Cardioid, Super Cardioid and Bidirectional.

This is a typical frequency response curve or graph.

Digital tracks can be made of live audio performance with special interface. Also individual track of every performer is recorded as per performer’s availability to record. To do this a pilot track is played on an electronic key board or by harmonium with clicks or raw rhythm. While listening to that track each performer alone performs given part and walks out. After processing each track for audio quality enhancement, all tracks can be mixed as a final mono or stereo audio clip for publication or converted to any selected format. Audio format can be aiff files for audio CD at 44.1 kHz sample rate, or MP3 or any suitable format.

There are many hard ware / software available free or for a price. I used Peak and Bais software bundled with Motu interface working good on my Mac computer. Then I started using Audacity v1.2.6 as free software on my assembled Windows machine with Vista OS, because the people are not ready to pay for my efforts.

These are few samples of Marathi Bhajans, some vocal tracks are recorded on Sony DSR 200 DVCAM camera with two Audio Technica mics, edited by Peak and Mixed with Bais. All the other instruments are recorded as individual  tracks, then enhanced or processed and published as aiff files for CD audio quality at 44.1 kHz sample rate. Here it is MP3 processed by Audacity from the original file.

This song tracks were recorded in my living room from start to finish the Bhajan was complete as a stereo track in total 4 hours. Govind

This song was without any rhythm. I played the rhythm track only on a Dagga  and mixed it with the original track. The music director approved it with pleasure. Vahe Vara

This vocal Track was recorded by Sony camera with harmonium played by the artist himself with a background rhythm but not recorded. the recording was done in his living room. Afterward the Dholki was added played by a professional. Tantradyani

*** all images are from Google image search ***

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Posted by on May 21, 2011 in My Audio



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