AUdIoCoUrSeS
Joined: 31 Oct 2002
Posts: 2014
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| Week 5 |
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Error handling systems
1. What is the sub-code area on a DAT tape used for?
2. Explain the principles of predictive coding.
3. How does the data buffer of a Minidisc player facilitate editing?
4. What are the basic principles of masking?
5. Explain the the following, with reference to a HD editing system
(a) disc access time
(b) EDL
(c) Disc data bandwidth
6. Explain briefly the problems of transferring data files between different proprietors HD systems.
7. Explain the principles used in editing on a single Mini Disc system.
8. Describe and explain the following error handling systems
• Causes of errors
• Minimisation of the consequences of errors
• Error detection
• Error correction
• Error concealment
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Mon Apr 02, 2007 7:41 am |
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resol69
Joined: 31 Dec 2002
Posts: 69
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| from nancy |
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1. What is the sub-code area on a DAT tape used for?
Sub Code areas allow extra information to be recorded with the audio . Sub-code areas currently used include:
• A-time, which notes the time taken since the beginning of the tape
• P-time, which notes the time taken since the last Start ID.
• Start ID marks the beginning of each recorded section. Start IDs become program numbers.
• Skip ID tells the machine to go directly to the next Start ID or program number.
• End ID marks the end of the recording on the tape. The End ID will cause the tape to stop, even if you are recording. In some cases you may want to use and END ID to protect material further along the tape, but unlike other ID's, adding and End ID will cause some erasure of sound.
All three IDS can be erased. Shuttle or skip to the ID you want to erase. The ID will illuminate on display. Press the ID Erase button. The tape will rewind to the beginning of the ID and begin the process of erasure. Press the RENUMBER button to resequence the program numbers if desired.
2. Explain the principles of predictive coding.
Predictive coding is a form of lossless compression. PC analyzes the speech signal by estimating the formants, removing their effects from the speech signal, and estimating the intensity and frequency of the remaining buzz. The process of removing the formants is called inverse filtering, and the remaining signal after the subtraction of the filtered modeled signal is called the residue. The numbers which describe the intensity and frequency of the buzz, the formants, and the residue signal, can be stored or transmitted somewhere else. Predictive coding synthesizes the speech signal by reversing the process: use the buzz parameters and the residue to create a source signal, use the formants to create a filter, and run the source through the filter, resulting in speech.
Because speech signals vary with time, this process is done on short chunks of the speech signal, which are called frames; generally 30 to 50 frames per second give intelligible speech with good compression.
3. How does the data buffer of a Minidisc player facilitate editing?
The date buffer allows for Rehearsal Mode. Holding the track mark button down for a few seconds enters rehearsal mode, allowing fine +/- adjustment of the track mark's position. So you can preview the editing process to get tracks to begin and end just exactly where you want.
4. What are the basic principles of masking?
Masking is basically one sound covering up another sound, so you don't hear it. In some cases an otherwise clearly audible sound can be masked by another sound. For example, conversation at a bus stop can be completely impossible if a loud bus is driving past. This phenomenon is called masking. A weaker sound is masked if it is made inaudible in the presence of a louder sound. The masking phenomenon occurs because any loud sound will distort the Absolute Threshold of Hearing, making quieter, otherwise perceptible sounds inaudible.
Different types of masking:
If the two sounds occur simultaneously and one is masked by the other, it's called simultaneous masking or frequency masking. The tonality of a sound partially determines its ability to mask other sounds.
If a weak sound emitted soon after the end of a louder sound is masked by the louder sound, it's called forward temporal masking. The opposite effect, when weak sound just before a louder sound can be masked by the louder sound is called backward temporal masking.
5. Explain the following, with reference to a HD editing system
(a) disc access time is the total time required for the computer to process the data request from the processor and then retrieve the required data from a storage device.
(b) EDL an Edit Decision List (EDL) is simply a list of desired takes and edits that will be used from the master recording, along with notes on where the cuts and edits will be performed. Hard disk systems use non-linear editing which is non-destructive. The original sound files remain intact, any editing only exists as a set of instructions that replay parts of the original sound files and overlay any signal processing. The files are identified with 'out' and 'in' edit points.
c) Disc data bandwidth is how fast data can be sent to and from the hard disk. Digital bandwidth is measured in bytes or bits per second.
6. Explain briefly the problems of transferring data files between different proprietors HD systems.
Because edit decision lists will be written differently for each system, you mix (session file) may not be carried over to the new system, or read on the new system incorrectly. You will either have to fix the errors that come up or do it again. Also, if going from a high quality system to a lower quality, or slower system, the new system may not be able to read the file correctly.
7. Explain the principles used in editing on a single Mini Disc system. ?All editing is based on the use of “tracks�. These tracks, like those on a Compact Disc are sections of the total disc recording which are marked as beginning and end points these track start and end points can be placed anywhere on a recorded section of disc, irrespective of the recording. All editing on Mini Disc is performed by manipulating tracks and track markers. Most Mini Disc machines will allow the following operations to be performed:
• Erase track marker (sometimes called combine tracks)
• Insert track marker (sometimes called divide tracks)
• Erase track
• Move track
• Undo
8. Describe and explain the following error handling systems
Causes of errors
There are two typical types of error:
Burst error: the loss of a number successive samples caused by signal loss. Signal loss could be the result of tape drop-out, electrical spike or damaged disc.
Random error is the loss of a single sample in a random position and is usually caused by noise or poor signal quality.
• Minimisation of the consequences of errors - The consequences of errors (what we hear) can be minimized by interleaving or redundancy.
Interleaving breaks up burst errors into smaller random errors that can be more efficiently corrected. Interleaving is achieved from recording samples out of their normal sequence. Taking columns from a memory which was filled in rows is one process of interleaving. Another is to shuffle the sample sequence to separate odd and even numbers, this way only either even or odd numbers will be lost. Upon replay the process is reversed (de-interleaved) putting samples back into their regular sequence.
Redundancy involves the recording of data in more than one place. All audio is recorded twice, with the odd/even technique reversed on the second recording. The result is two separate interleaved recordings. If their is a burst error in the odd numbers, this will affect the odd numbers of both recordings. As the odd/even numbers are reversed on the second recording, the lost data of the first recording is actually in the even numbers of the second.
Error detection - Cyclic redundancy check (CRC) codes are used in many systems to detect errors on replay. These codes are calculated from the original data and recorded along with that data.
Error correction - There are four types of error correction:
True error correction recreates erroneous samples perfectly. These corrected samples are indistinguishable from the originals, therefore sound quality is unaffected.
Interpolation: used when the error rate exceeds the limit of perfect correction. An interpolated sample value is a mathematical average of the sample before and the sample after. This process is also known as concealment or averaging. Hold: used when the last correct sample is repeated. A system will usually only hold for a few samples before muting.
Muting is the last option of error correction. When the error correction system is totally overwhelmed, the audio output will be muted for the duration of the error. Sometimes it would be preferred to hear the error, which may be a loud click.
Error concealment; an error is detected but it is too severe to be corrected. Missing data becomes a “guess” based on the surrounding data and the result hopefully will be inaudible. However, if you ever get chance to see a CD player that has correction and concealment indicator lights, you will notice that an awful lot of concealment goes on just to play an average disc. How well concealment is done is one of the factors that make different digital systems sound different. |
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Sun Apr 15, 2007 2:24 pm |
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