Numbers to know

Student reference card. Learn these. There is a five minute verbal quiz at the top of every session, and you may bring this page into the calculate station of the practical exam.

Print it. Fold it. Put it in the pocket of the jacket you wear to work.

Block 1: Data rates (learn before the production visit)#

The ranking, and you should be able to say it in your sleep: clock < control < audio < video.

The arithmetic:

audio  =  sample rate  ×  bit depth  ×  channels
video  =  width × height × frame rate × bits per pixel

Block 2: The machine (learn before Class 3)#

| Light in glass fibre | about 5 µs per km, so 5 ms per 1,000 km, each way | | Sound in air | about 3 ms per metre | | A frame at 25 fps | 40 ms | | Camera to LED wall, typical | 3 to 7 frames, so 120 to 280 ms | | Precedence window | 5 to 35 ms: two arrivals fuse and localise to the first | | Delay tower setting | the distance from the PA, in ms, plus 10 to 15 |

Bits to bytes: divide by 8. A 2.5 Gbit/s video stream is about 312 MB/s of storage throughput. Getting this the wrong way round is the most common mistake in the exam.

Block 3: Addressing and the network (learn before Class 4)#

The rule: two devices can talk directly only if the network part of their addresses is identical under the mask.

Three symptoms to recognise instantly:

• 169.254.x.x → DHCP was expected and there is no DHCP server.
• Intermittent, confusing, both devices misbehaving → duplicate IP.
• Works fine, then stops several minutes later → multicast, snooping on, no querier.

The nine bit values (learn these first)#

The prefix table (learn these second)#

The subnet method, four steps#

1. Block size      = 256 - (last non-zero mask octet)
2. Find which block the address falls in, counting in steps of the block size
3. Network = first address in the block.  Broadcast = last address in the block.
   Usable  = everything between them.
4. Usable hosts    = 2^(32 - prefix) - 2

The OSI layers#

Diagnose from the bottom up. Link light, then VLAN, then IP, then port, then software.

VLAN numbers#

Block 4: Protocols (learn before Class 5)#

The DMX refresh calculation, learn to reproduce it:

512 slots × 11 bits = 5,632 bits
5,632 ÷ 250,000 = 22.5 ms
plus break and mark ≈ 23 ms per frame
1 ÷ 0.023 ≈ 44 frames per second

Block 5: Media over IP and perception (learn before the practical exam)#

Why the asymmetry: thunder always follows lightning, so we are built to accept sound arriving after picture. Sound arriving before picture is unnatural and we catch it roughly three times faster.

The two things you will be asked to do, in every assessment#

1. Data rate maths. Will this fit down this wire?

channels or pixels  ×  rate  ×  depth  =  bits per second

2. Latency budget. Add every stage from trigger to eardrum or retina. Compare it against a perceptual threshold. State pass or fail, and say which threshold you used.

The two definitions people get wrong#

Latency is delay. If it is constant, you can measure it and compensate for it. A delay tower is a latency compensation device and nobody calls it a fault.

Jitter is delay that varies. You cannot compensate for a number that will not sit still. Your only defence is a buffer, and a buffer costs latency.

Jitter is what breaks shows.