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December 2002 Safety Tips While On Tour - Issue: Dec. 2002
It’s always a good idea for at least one member of the crew to be trained in first aid techniques. It may as well be you... In the meantime, there are self-help and precautionary steps that everyone involved in PA work can take. First of all, here are some quick tips on preventing back strain when lifting and moving heavy gear. -- First of all, if the object looks like it’s too heavy for one person to lift, just get some help. Forget macho – how macho is it to be laid out in a hospital bed in traction? -- If you are going to tackle it yourself, think of the following word: BACKUP. It stands for the following: If you’ve wrenched something, bashed something, cut something or you’re just generally feeling poorly, think on this: Hospitals are no friends of minor complaints, and in some countries treatment is uncertain and expensive. Or you might be stuck on a festival site, feeling ill, but too badly needed to leave. Or say you witness a fellow crewmember lying injured, and there’s no one else to help them... Assistance could be at hand, in the form of a book like The Family Guide to Homeopathy by Dr. Andrew Lockie, which has some sound advice on first aid and ‘bodily disorder’ treatment, using homeopathic remedies where appropriate. The remedies listed can be safely self-prescribed and are low-cost. A basic first aid kit of about 20 types of ‘remedy’ pills, one tincture and five creams covers most situations – from burns, crush injuries, weird food poisoning, sprains, smog fumes and all manner of other minor troubles that stop you from giving 100 per cent. Of course, if the injuries are plainly serious, or first aid doesn’t ease matters fairly quickly, or symptoms worsen, immediate hospitalization is advisable. This article is reprinted with permission from The Live Sound Manual, published by Backbeat Books, www.backbeatbooks.com. All information is copyrighted and cannot be reprinted without the permission of the publisher.
Staying In Synch -- Part I: Word Clock Explained - Issue: Dec. 2002
by Bob Snelgrove Everyone using more than one piece of digital audio equipment should be concerned about the quality of their studio's Word Clock. This article will explain the critical role that Word Clock quality and distribution plays in the digital audio environment and the audible effects that poor clocking has on digital music systems. Examples of common mistakes and suggestions for proper hook-up will be made. I need to start off by saying that when it comes down to quality of sound, I tend to approach the wonders of digital audio with a healthy degree of caution. I love my CD player and I personally own and enjoy lots of other digital audio toys. Nevertheless there are still many problems with the digital representation of audio that have yet to be solved. All of these problems relate to digital audio’s sonic accuracy and transparency. Timing A sample clock determines when these sample instants occur. All digital audio devices have some form of a sample clock to control their internal sample rate or sampling frequency. In a studio where we integrate many different pieces of equipment that all depend on their own clocks to function we will invariably have sample instants taking place at different times unless we synchronize all the clocks in each piece of equipment and tie the timing of these events together. This synchronous timing is required because unlike analog audio, digital has a discrete time structure consisting of individual samples. Successful communication between different digital audio devices or the mixing of different digital audio signals together will fail if each device is not producing their bits of data in precise co-ordination with each other. Poor quality timing between multiple digital audio devices or the improper distribution of those timing signals will result in non-synchronous operation and the result will be the creation of random and highly audible artefacts, often described as clicks, pops or glitches. There are two standard timing signals used to synchronize the internal sample clocks of digital audio equipment. The first, commonly used in large post-production and broadcast facilities is the AES3 Digital Audio Reference Signal or DARS for short. This bi-phase signal’s carrier is exactly the same as a balanced AES3 signal but no audio data (digital zeros) in the data stream. An XLR sync input can usually be found on pieces of high-end audio equipment and workstations. DARS is distributed the same way as AES3, via balanced 110-ohm digital audio cable and AES3 XLR type connectors. There are two things that make the DARS or Audio Black signal particularly attractive. The first is its high frequency of operation, which is between 2 and 3 Mbits per second. The second is the fact that the professional AES3 interface is balanced and ground isolated making it relatively immune to induced noise, which can be a major source of jitter. The most common clock distribution method however is Word Clock. The Word Clock waveform is a simple unbalanced square wave. Word Clock is designed to be distributed on 75-ohm, unbalanced coax cable terminated with BNC connectors. In order for synchronous operation to take place all digital audio devices must be fed one of these timing signals from a master reference Word Clock time base. These are typically referred to as Word Clock generators and sometimes as synchronizers depending on the functionality they provide. Great care must be taken to carefully distribute these Word Clock signals to each piece of digital equipment in the studio or the timing signals will be degraded and audio quality will suffer. Word Clock Generation A special case of declining interest and usefulness is Digidesign’s proprietary SuperClock at FsX256 or a frequency between 11.289 and 12.288 mHz. Because different manufacturers design their products to accept different multiples of Word Clock base frequencies, all multiples from one to four must be supported, and the master word clock generator must be able to produce different Word Clock frequencies from different outputs simultaneously. It is important to note that not all BNC word clock outputs are created equal. When distributing Word Clock signals, isolation and proper source impedance from each output BNC is important. If the outputs are simply fed from one low impedance source then it will be impossible to correctly terminate a line and a single bad cable or poor connection will reflect back and compromise the performance of every other line. This is one condition that would also create jitter. Join us next issue, where we continue are look at synchronization in the digital world, tackling the topic of jitter. Bob Snelgrove is the President of GerrAudio Distribution and the Canadian Product Specialist for Audio Precision test instrumentation.
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