AUdIoCoUrSeS
Joined: 31 Oct 2002
Posts: 2014
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| Week 5 - Error Handling |
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Error handling systems
With this weeks questions i'm looking for much more elongated answers and details. Look to answer these very fully, particularly the last question.
1. What is the sub-code area of the 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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Sun Oct 03, 2004 6:09 pm |
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iNSTiNCT2765
Joined: 05 Nov 2003
Posts: 60
Location: Denmark |
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Week 5 – Error Handling
1. What is the sub-code area of the DAT tape used for?
The sub-code area of a DAT tape is used to store the Start, Skip and End ID’s, Program Numbers and various date and time stamps. The time stamps include:
A-Time (absolute) – starts at the beginning of the tape and counts sequentially through the recording.
P-Time (programme) – provides a timer for each Start ID.
Timecode information is also stored in the sub-code of the DAT tape. Any frame rate timecode is transposed into the DAT’s internal frame rate of 33.333 fps. When replayed, the DAT’s internal frame rate is regenerated into the user determined output frame rate.
2. Explain the principles of predictive coding.
Predictive coding is a data reduction method, which has a ‘predictor’ that has knowledge of typical audio signal behaviour. The ‘predictor’ looks at the preceding audio signal and attempts to determine what will happen next and due to the repetitive nature of audio signals, the prediction is usually very accurate. The difference between the original signal and the prediction is then recorded as the data reduced signal. To regenerate the signal, a decoder must use the same ‘predictor’ knowledge and this is usually done to an accuracy of 98%. This accuracy is completely dependant on the predictor algorithm. To improve the precision of the system, a multi-band coding technique is also being used. This means splitting the signal into four frequency bands and applying separate predictors for each. One thing to be taken into consideration is that the encoder and decoder of the system must use the exact same predictor. Therefore, both have to be updated in order to ensure no signal degradation. The predictive coding system provides a 4:1 data reduction.
3. How does the data buffer of a Minidisc player facilitate editing?
Since the data stored on the Minidisc is non linear, it is accessed in the same way as data on a hard disk. Data is stored in segments so editing is made easier. If you have 10 tracks on the Minidisc and erase track 4, which was a 3 minute song and then decide to record a new song that is 4 minutes long, the first three minutes of the track will be stored where track 4 used to be and the rest will be placed at the next available empty segment, which is after track 10. When playing back the new track, the read buffer keeps the track seamless while the Minidisc accesses the first segment and then the second.
4. What are the basic principles of masking?
When the ear hears two tones with the same dominant frequencies, the louder tone will make the softer tone less sensitive to the ear and it will be masked so only the louder tone is audible. Lossy data compression takes advantage of this phenomenon, since the human ear does not hear the masked tones anyways, they don’t need to be coded, therefore saving storage space.
5. Explain the following, with reference to a HD editing system
(a) Disc access time – data on a hard disk is recorded as a series of concentric tracks and to access them, the pick up must first locate the particular track and then the disk has to spin so the data can be read. This process determines the disc access time and the longer it takes, the greater the delay is.
(b) EDL – this stands for edit decision list and is a record of a sequence of edits. The system can then scan this list and store the information in memory when executing precomputed fades.
(c) Disc data bandwidth – this is the rate at which data can be recorded and played on and off a hard disk respectively.
6. Explain briefly the problems of transferring data files between different proprietors HD systems.
Since different HD systems may use different forms of data reduction or compression formats, transferring data between them may not be an option if one HD system doesn’t have the specific format of data reduction available. Then the HD won’t be able to read the audio transferred. Session files and plugins from one HD system may not be compatible with another system and therefore the project data won’t function properly on the other system.
7. Explain the principles used in editing on a single Mini Disc system.
The Minidisc system uses a Table of Contents data structure that links sections of audio data that is scattered around the disc into one continuous stream. This allows tracks to be moved, deleted, segmented and combined with an edit point accuracy of about 12 ms. The space a deleted track used to occupy can be filled with new recorded material.
8. Describe and explain the following error handling systems
• Causes of errors – with digital audio, dropouts may occur, which results in data not being able to be read. They can be caused by damaged hardware such as a broken or malfunctioning hard disk. A CD with smudges or scratches on it can prohibit the laser from reading the data causing errors.
• Minimisation of the consequences of errors – error probability can be minimised by interleaving data. Interleaving is the process of scattering data around the storage medium so if a large section is damaged, it results in many small manageable data losses that can be recovered using error correction.
• Error detection – the sum of the numbers of a word can be added up and stored with the data itself. Then when a system reads the data, they can cross check with the stored numbers to check if they are the same. This is called check sum. If the data being read doesn’t add up to the check sum number, an error will occur.
• Error correction – using additional data recorded onto the medium with the digital audio, the data can be reconstructed and the errors corrected. A type of this is the Reed-Solomon error correction coding system, which is used in systems including CD’s, DVD’s, barcodes, and satellite communications.
• Error concealment – this is a technique used to reduce the effect of a digital error if the error cannot be corrected using error correction. It involves making a smooth transition from the last good block and first good block before and after the error respectively. This is done using some form of crossfading. This is the reason why a digital copy may not be an exact duplicate of the original master. |
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Sat Oct 09, 2004 10:03 am |
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