Digital Audio. Amplitude Analogue signal

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1 Digital Audio The sounds that we hear are air molecules vibrating in a wave pattern. These sound waves are measured by our ear drums and processed in our brain. As computers are digital machines, sound must be converted in a digital form if it is to be recorded and manipulated in a computer. Sound is represented as waves. These waves have an amplitude (height) and a repeat cycle (time). Amplitude Analogue signal Time Sampling rate To represent the varying values of a soundwave, it s height must be measured at regular intervals and the measurements given binary codes. This process is called Sampling and the number of samples taken in a second is called the sampling rate. The higher the sampling rate, the better quality sound will be produced. Recording Digital Audio Audio can be recorded into a computer system using a microphone or transferred from an internal or external player. The audio ports at the back of the computer system are used to connect a microphone or an external player such as a tape cassette or record player. Microphone (Pink) Speakers (Green) Eternal Player (Blue)

2 A microphone has a membrane similar to an eardrum which vibrates as the sound waves strike it. Theses vibrations are translated into electrical signals - the higher the vibration, the higher the electrical signal. When the sound from the microphone or player reaches the sound card, an analogue to digital convertor (ADC) built into the card converts the sound to digital values. The number of values taken is set by the sampling rate. In the diagram below, the sampling rate is represented by the dots on the analogue wave. A number of factors effect the quality of the digital recording: Amplitude Analogue signal Time Sampling rate Sampling rate - the number of times that measurements (samples) are taken from the wave every second. This is measures in hertz (Hz - samples per second). Audio CD s are recorded at 44KHz and as a result, most audio software uses this value as the default. DVD has adopter a standard of 96 or 192 KHz. Sample format - this is the number of bits (binary digits) used to represent each sample. Audio CD s use 16 bits per sample and this is the default used by audio software. 24-bit and 32-bit samples can also be used, but this will result in larger audio files. Compression - as with data files, audio files can be compressed to reduce file size. This is not just a matter of removing repetitive elements within the file, it involves sophisticated methods of getting the full sound range using fewer bits for each sample. To play sound on the computer, a digital to analogue convertor (DAC) built into the sound card converts the digital values to electrical pulses and sends them to the speakers. Here, an electromagnet in the speaker housing causes the electrical pulses to be translated into vibrations of the speaker membrane and as a result, sound waves are produced and transmitted.

3 Digital Audio Software There are a number of software applications available for digital audio editing. Most of these are quite expensive and can be used to record, edit and mix audio. Examples include Adobe Audition, Sony Sound Forge and Steinberg Cubase. Audacity Audacity is a free to use multi-track digital audio application. It is a fully featured crossplatform editor, mixer and recorder. It is available for download at: File Formats Audio tracks can be imported into audacity in almost all file formats. Files can be saved in the following formats: Audacity project format (aup) - this is the native audacity file format. It opens and closes files very quickly. It is incompatible with all other audio software and is used during project creation. The finished project is then saved to another file format. Windows wave format (wav) - this is the default audio file format on windows computers. It is an uncompressed format. Audio interchange file format (aif) - this is the default audio file format on Apple MAC computers. It is also an uncompressed file format. MPEG3 (mp3) - this file format uses compression and is commonly used to store music. Compression rates of up to 10:1 are possible with little discernible loss of quality. Audacity can import MP3, but a plug-in is necessary to export MP3. Ogg Vorbis (ogg) - designed as an alternative to MP3, the compression rate is similar to MP3, but the quality is better. It is a supported by most digital audio software and is gaining in popularity as it is totally patent free, unlike MP3. The Audacity Interface To Open Audacity, click Start, All Programs and choose Audacity from the list

4 When Audacity is opened, the work area is empty. Audio files can be opened in audacity in the following ways: The File menu, Open dialog is used to: Open an existing Audacity project Open a single audio file for editing The Project menu, Import Audio dialog is used to: Add an audio file to an Audacity project. This is the method used for multi-track editing. The interface has a number of areas: The Menu Bar The menu bar contains the drop down menus containing the complete set of commands available in Audacity. The menus are: File menu - contains commands for opening and saving projects and for exporting audio files in various file formats Edit menu - contains the commands for selecting and editing audio tracks. View menu - this menu contains the zoom controls

5 Project menu - used for adding tracks, importing audio etc Generate menu - this menu contains commands for generating silence, white noise etc Effects menu - contains effects commands such as fade in or out, amplify, noise removal, echo etc Analyse menu - contains advanced tools for finding the musical beat etc. The Control Toolbar The control toolbar is divided into two sections: Editing tools Audio control tools The editing tools are used for zooming, selecting and editing tracks: Selection Tool The selection tool is used to select a track or a portion of a track for editing. In the example below, the Tank.wav file is opened. The selection tool is used to select a portion at the end of the waveform. The Fade Out command is chosen from the Effects menu and the result is seen below. Instead of a sudden end to the sound, it will fade to silence over the selected time. Zoom Tool The zoom tool is used to zoom in and out of a waveform. The left mouse button zooms in and the right mouse button zooms out. Zooming if necessary when editing a waveform.

6 The Draw Tool The draw tool is used for modifying individual samples within the waveform. The waveform must be zoomed sufficiently to see the samples as dots along the profile. Simply choose the tool and drag the samples up or down as shown below. Samples The Envelope Tool The envelope tool is used to change the volume of a waveform or selection. When the tool is chosen, blue lines appear above and below the waveform. By clicking the blue line at points, the waveform amplitude can be increased or decreased. with the corresponding reduction or increase in volume. The Time Shift Tool The time shift tool is used in multi track mode to move tracks relative to each other. Move the cursor to the beginning or end of the track and the cursor changes to the double-arrow to indicate time shift. Drag the track to the required position relative to the other track(s).

7 The two tracks below will play simultaneously The time shift tool was used to shift the lower track and the tracks now play in sequence Multi Tool Mode This tool gives access to the other five editing tools automatically depending on the location of the cursor: Move the cursor over the ruler and it changes to the zoom tool Move the cursor to the beginning or end of the waveform and it changes to the time shift tool Move the cursor over the waveform and it changes to the selection tool etc. Audio Control Tools These tools are used to control the recording and playing of audio tracks or projects. Skip to start - moves the cursor to the beginning of the project. Skip to End - moves the cursor to the end of the last track Play - plays or loops the project from the current cursor position Record - starts recording a track from the current cursor position Pause - temporarily stops recording or playback. Stop - stops recording or playback

8 The Mixer Toolbar The mixer toolbar has three controls: The playback volume is the volume is used by the system to play the audio file The recording volume is the volume at which the audio track is recorded Playback volume Recording volume Input source The input source drop down menu is used to select the device used for recording: Microphone - records using a microphone connected to the microphone jack (pink) on the computer Line in - records from an external device connected to the line in jack (blue) on the computer Stereo mix - records in stereo anything played through the sound card. This includes audio from the internet or the internal CD or DVD player. Mono mix - as above, only in mono The Edit Toolbar The edit toolbar is a shortcut toolbar and is used to perform edits on a selected waveform of part of a waveform. Cut - removes the selected waveform or part of to the clipboard Copy - copies the selected waveform or part of to the clipboard Paste - pasts the contents of the clipboard at the current cursor position Trim - removes the area outside the current selection Silence - adds silence to the current selection

9 Undo/Redo - as in any application Zoom - plus, minus, fit selection in window and fit project (multi-track) in window Meter Toolbar The meter toolbar monitors the input and output levels. The playback meter is in green while the recording meter in in red In stereo recording or playback, the L and R meters are for the left and right channels The light areas indicate the average levels while the darker areas indicate the peak levels The peak lines to the right show the maximum level in the previous three seconds The lines at the extreme right show only if clipping is detected. Clipping is where the audio is too loud and distortion has occurred. Recording should be stopped and rerecorded at a lower level Track Menu Each track in a multi-track project will have a track drop-down menu. The menu shows a series of commands that effect only the individual track. The commands include: Name - name or rename the track Move track - move the track up or down the project Waveform - show the waveform in various formats Mono - convert a stereo track to mono Left channel - force the track to use the left speaker only Right channel - force the track to use the right speaker only Make stereo - joins the track to the one below it to make a stereo track Split track - splits a stereo track into two mono tracks which can be edited individually Set format - choose between 16, 24 and 32 bit format Set rate - set the number of samples per second for the track

10 Status Area The status area shows: The current tool selected, the current sampling rate and the current selection Preferences Preferences are found in the Edit menu. There is an extensive range of preferences, but only a few need be set. In the Audio I/O tab, the Recording and Playback devices ca be set. These should be set to the system Sound Card in each case. The number of channels used to record is also set here. This will be mono or stereo. In the Quality tab, the Sampling rate and the Sampling format are set

11 MP3 and Audacity There is no native MP3 encoding built into Audacity. To export audio files as MP3, a third party utility called Lame is required. Lame can be downloaded as a zip archive from: Click the link on the site as shown below. Click Save in the dialog box Save the file to a suitable location Extract the file using WinZip or the Windows extraction utility

12 Copy the extracted folder to the C: drive In Audacity, open a wav file from the Windows, Media folder In the File menu, close Export as MP3 Choose a Name and Location for the file

13 Click the Save button. The Export MP3 error window appears Click Yes in the window Browse to the location of the lame.dll file in the folder copied to the C:drive Select the lame.dll file and click Open Click OK in the Edit ID3 Tags window The file is saved as an MP3 file. This process needs to be performed only once. Audacity now has MP3 export capability