Multimedia Hardware Platforms
MOD002556
17/03/2014
13:00-16:00
http://www.theregister.co.uk/2011/08/18/white_space/
Analysis The white spaces are filling up around Cambridge, so El Reg went up to the Fens to talk to the companies responsible about what white space is for ... and why they're sticking their masts on pubs.
Ofcom issued an
experimental white space licence permitting low-power transmissions within an 80MHz-wide band around Cambridge in June, and since then local companies have been busy deploying white space radios for experimentation. We dropped in to see three of them, and discovered some very divergent ideas about how white space will be used, though there was a general consensus that Local TV was just a bad idea.
Two of the companies we visited are already running white space links around 6km, and getting a reliable 3Mb/sec in an 8MHz channel. The third has covered the whole city in a white space network, intended to connect machines to machines and showcase the company's proposed standard for such communications.
White space radios operate in frequencies which are reserved for TV transmissions, but aren't being used locally thanks to the hugely overpowered radios used to broadcast television. A transmitter in London might reach to Cambridge (Crystal Palace does, just) so TV transmitters can't use the same frequency, but short-range devices can, and short range can be pretty long.
As no one in Cambridge is using the Crystal Palace transmissions to watch TV, there's no chance of those devices knocking out local TV reception, as long as they know where they are. The weak signal from the Palace can safely be ignored, if it can be picked up at all, and as long as everyone has their telly aerials pointed the right way (towards Sandy Heath) they won't pick up the local transmissions.
Rural broadband is the most obvious application of white space, and will probably be the first to be widely adopted. White space devices will have to consult an online database to check what frequencies are available locally, which is ideal for an access-point topology where the hub can check online and then tell client devices which frequency to use.
TTP, which already has a 5.5km link running at up to 4Mb/sec, reckons that such access will eventually be wholesaled nationally, with BT being the obvious provider. The
recently approved 802.22 standard should fit such applications well, though both TTP and Cambridge Consultants (who have their own 6km link running at 2Mb/sec) reckon the database needs to be a two-way affair if it's going to work properly.
Ask and you shall receive
The current thinking for white space is that devices will submit their location and receive a list of the available channels in their area. But even at this stage, it is becoming obvious that static data will not be enough when a TV transmitter can stretch 60 miles or more depending on the weather, so both companies expect some nodes to report back to the database on local conditions, to the benefit of other white space users.
Quite how that works in practice remains to be seen: Ofcom has yet to decide who will run the UK database, and how, though an announcement is expected soon. The point here is that a one-way relationship isn't going to allow efficient use of the available radio frequencies.
Not that the database will ever be redundant – hidden transmitters cannot be sensed but could be interfered with – but using a database alone would require worst-case allocations, which would reduce the white space available, so a combination effort seems most practical.
Everyone involved in white space sees the technique as a key driver for cognitive radio technology. Cognitive devices are able to sense and avoid frequencies being used, and are still in their infancy. DECT phones will scan a handful of channels to avoid other DECT devices, and a few Wi-Fi routers can automatically select the most-empty channel, but that's detecting a common standard operating within specific bands. Cognitive white space devices will be expected to detect all kinds of use across very wide frequencies.
But if that can be achieved, and been proven in the white spaces, there seems little reason not to apply it elsewhere. Ofcom sincerely follows its remit to promote efficient use of radio spectrum, generally using auctions. This is based on the idea that whoever pays the most has greatest incentive to fill the bandwidth, but the regulator is always open to suggestions on better ways.
Sensing becomes even more important when one starts looking at some of the more innovative applications of the white spaces. Pop-up networks could provide connectivity across a festival or event site, providing decent bandwidth at a very low price. The 802.22 working group reckons it can fit 22Mb/sec over an 8MHz channel, which is probably optimistic, but there's nothing to stop channels being combined.
An analog-to-digital converter (abbreviated ADC, A/D or A to D) is a device that converts a continuous physical quantity (usually voltage) to a digital number that represents the quantity's amplitude.
The conversion involves
quantization of the input, so it necessarily introduces a small amount of error. Instead of doing a single conversion, an ADC often performs the conversions ("
samples" the input) periodically. The result is a sequence of digital values that have converted a continuous-time and continuous-amplitude
analog signal to a
discrete-time and discrete-amplitude
digital signal.
An ADC is defined by its bandwidth (the range of frequencies it can measure) and its signal to noise ratio (how accurately it can measure a signal relative to the noise it introduces). The actual bandwidth of an ADC is characterized primarily by its
sampling rate, and to a lesser extent by how it handles errors such as
aliasing. The
dynamic range of an ADC is influenced by many factors, including the resolution (the number of output levels it can
quantize a signal to), linearity and accuracy (how well the quantization levels match the true analog signal) and
jitter (small timing errors that introduce additional noise). The dynamic range of an ADC is often summarized in terms of its
effective number of bits (ENOB), the number of bits of each measure it returns that are on average not noise. An ideal ADC has an ENOB equal to its resolution. ADCs are chosen to match the bandwidth and required signal to noise ratio of the signal to be quantized. If an ADC operates at a sampling rate greater than twice the bandwidth of the signal, then
perfect reconstruction is possible given an ideal ADC and neglecting quantization error. The presence of quantization error limits the dynamic range of even an ideal ADC, however, if the dynamic range of the ADC exceeds that of the input signal, its effects may be neglected resulting in an essentially perfect digital representation of the input signal.
A
sound card (also known as an
audio card) is an internal
computer expansion card that facilitates the input and output of
audio signals to and from a computer under control of computer programs. The term
sound card is also applied to external audio interfaces that use software to generate sound, as opposed to using hardware inside the PC. Typical uses of sound cards include providing the audio component for multimedia applications such as music composition, editing video or audio, presentation, education and entertainment (games) and video projection.
Sound functionality can also be integrated onto the
motherboard, using basically the same components as a plug-in card. The best plug-in cards, which use better and more expensive components, can achieve higher quality than integrated sound. The integrated sound system is often still referred to as a "sound card".
Conventional PCI, often shortened to just
PCI, is a
local computer bus for attaching
hardware devices in a
computer. PCI is an initialism of Peripheral Component Interconnect and is
[2] part of the PCI Local Bus standard. The PCI bus supports the functions found on a
processor bus but in a standardized format that is independent of any particular processor's native bus. Devices connected to the PCI bus appear to a bus master to be connected directly to its own bus and are assigned addresses in the processor's address space.
[3][page needed] It is a
parallel bus, synchronous to a single
bus clock.
Surround sound is a technique for enriching the
sound reproduction quality of an audio source with additional
audio channels from
speakers that surround the listener (
surround channels), providing sound from a 360° radius in the horizontal plane (2D) as opposed to "screen channels" (
centre, [front] left, and [front] right) originating only from the listener's forward arc.
Surround sound is characterized by a listener location or
sweet spot where the audio effects work best, and presents a fixed or forward perspective of the sound field to the listener at this location. The technique enhances the
perception of sound spatialization by exploiting
sound localization; a listener's ability to identify the location or origin of a detected sound in direction and distance. Typically this is achieved by using multiple discrete
audio channels routed to an array of
loudspeakers.
There are various surround sound based formats and techniques, varying in reproduction and recording methods along with the number and positioning of additional channels.
MIDI carries event messages that specify
notation,
pitch and velocity, control signals for
parameters such as volume,
vibrato,
audio panning,
cues, and
clock signals that set and synchronize
tempo between multiple devices. These messages are sent to other devices where they control sound generation and other features. This data can also be recorded into a hardware or software device called a
sequencer, which can be used to edit the data and to play it back at a later time.
[2]:4
Advantages of MIDI include compactness (an entire song can be coded in a few hundred lines, i.e. in a few
kilobytes), ease of modification and manipulation and choice of instruments.
[3]
Digital audio refers to technology that can be used to record, store, generate, manipulate, and
reproduce sound using audio signals encoded in
digital form. Following significant advances in digital audio technology during the 1970s, it rapidly replaced
analog audio technology in most areas of sound production, sound engineering and telecommunications.
Digital audio systems may include
compression,
storage,
processing and
transmission components. Conversion to a digital format allows convenient manipulation, storage, transmission and retrieval of an audio signal.
VoiceXML (
VXML) is a digital document standard for specifying interactive media and voice dialogs between humans and computers. It is used for developing audio and voice response applications, such as banking systems and automated customer service portals, that are developed and deployed in an analogous manner to the interaction between
web browsers, which render
Hypertext Markup Language (HTML) in visual applications, and the servers that deliver them. VoiceXML documents are interpreted by a
voice browser. In common deployment architectures, banks of voice browsers are attached to the
public switched telephone network (PSTN) which interact with users via the telephone network.
