Electrical networks for emulating the response or co-axial transmission cable to serial digital video signals

An electrical network emulates the transmission characteristics of a video signal cable, allowing tests to be made as to the length to which the cable may be extended before video signal degradation occurs. The preferred embodiment emulates a Beldon.RTM. 8281 coaxial cable, used for the transmission of serial digital video signals, and comprises four sections. The sections are arranged to emulate cable lengths of 25 meters, 50 meters and 200 meters.

FIELD OF THE INVENTION 
The present invention relates to an electrical network and a method of 
testing the length of cable over which a signal may be transmitted. 
BACKGROUND OF THE INVENTION 
Many transmission standards are known for transmitting signals over cables. 
Some of these standards allow signals to be transmitted over very large 
distances, for example such as those employed in telephony. Many of these 
transmission standards involve the modulation of signals, so as to present 
a modulated signal to the transmission medium which can be transmitted 
over much longer distances through said medium. 
Over shorter lengths, for example, within a building or data processing 
facility, it is desirable to transmit signals in an unmodulated form, 
thereby avoiding the need for a multiplicity of modulating and 
demodulating devices. However, in this form, signal degradation is more 
likely to occur, as the lengths of cable increase until a critical point 
is reached at which the cable is too long for the signal to be 
transmitted. 
In such an environment, a common problem for technicians and engineers is 
that of determining the extent to which a cable may be extended before 
critical degradation does occur. A known technique for making this 
assessment consists of inserting lengths of cable into the circuit and 
assessing whether the equipment continues to function correctly with said 
additional length in place. A major problem with this approach is that 
even a relatively short additional length of cable can be quite heavy and 
bulky and to perform a suitable test, it may be necessary to connect 
several different lengths of cable. Thus, the problem is not essentially 
one of determining how the test may be performed but of physically 
implementing the test, given that cables may be less than perfectly 
accessible and the space available for making such tests may be limited. 
An environment where such a problem often occurs is in a television studio 
or video editing facility, in which digital video signals and, more 
recently, serial digital video signals, are transmitted between various 
processing devices, such as editing desks, recording equipment and effects 
machines etc. 
In accordance with the CCIR 601 standard, 10 bit parallel data may be 
converted to a serial bit stream consisting of 270 megabits per second 
transmitted by scrambling the information in accordance with an non-return 
to zero algorithm. Alternatively, composite video in NTSC or format, 
may be transmitted in accordance with the D2 standard at 144 megabits per 
second or 177 megabits per second respectively. 
As previously stated, the transmission of such signals over physical cables 
will result in signal attenuation and the level of attenuation will depend 
upon the quality of the cable and its length. However, the level of 
attenuation and signal delay also varies with the transmission frequency 
of the signal and, being digital in nature, the transmitted waveform will 
be made up of a plurality of different frequency components. Thus, these 
frequency components will tend to be attenuated and delayed by differing 
amounts, resulting in signal distortion. 
Being a digital signal, the degree of attenuation and distortion introduced 
by the cable will not result in appreciable signal degradation until a 
certain level has been attained, whereafter, the degradation becomes 
unacceptable and the resulting noise introduced to the signal results in 
an unacceptable number of errors being introduced. 
Thus, in assessing the extent to which the length of the cable may be 
increased, it is necessary to keep adding different lengths of cable and 
then determine whether an acceptable or an unacceptable level of 
degradation is occurs. 
SUMMARY OF THE INVENTION 
According to a first aspect of the present invention, there is provided an 
electrical network, characterised by being configured to have a frequency 
response over a frequency range of interest substantially similar to the 
frequency response of a predetermined length of cable, wherein said 
network emulates the transmission characteristics of said cable. 
Thus, the present invention provides a solution to the problem of 
increasing cable lengths for testing purposes. Rather than actually 
introducing a physical length of cable, the network emulates the 
transmission characteristics of the cable, thereby allowing tests to be 
made without introducing additional lengths of physical cable. 
In a preferred embodiment, the network emulates the characteristics of 
cable when transmitting coded digital signals. Thus, the emulation 
bandwidth may be limited to take account of redundancy in the transmitted 
signal, particularly when coded algorithms are being employed. 
In a preferred embodiment, the network emulates the response of cable at 
frequencies associated with the transmission of serial digital video 
signals. Preferably, processing means are provided for assessing the 
integrity of the serial digital video signal after transmission. 
According to a second aspect of the present invention, there is provided a 
method of testing the length of cable over which a signal may be 
transmitted, comprising the steps of assessing the level of degradation to 
the signal after transmission, characterized by 
applying said signal to an electrical network configured to have a 
frequency response, over a frequency range of interest, substantially 
similar to the frequency response of a predetermined length of cable, 
wherein said network emulates the transmission characteristics of said 
cable.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments are networks, passive or active, for emulating 
the response of lengths of cable over a frequency range of interest. The 
networks may have many applications but are particularly directed towards 
testing system, in that, an evaluation can be made as to the response of 
the system when an additional cable length has been inserted. The 
preferred embodiments will be described with reference to the transmission 
of serial digital video signals and, in particular, signals conforming to 
the CCIR 601 standard but it should be appreciated that similar techniques 
may be employed for emulating the transmission of any signal over any 
transmission medium. 
A serial digital video transmission system is shown in FIG. 1, in which a 
transmission cable is modelled by a network 15, having a characteristic 
impedance of 75 ohms. A transmitter consists of a signal source 16, having 
an internal impedance 17 matched to the characteristic impedance on the 
cable 15. At the receiver, the cable is terminated by an impedance 18, 
equal to the characteristic impedance of the cable, along with an 
equaliser 19. The purpose of the equaliser 19 is to compensate for the 
attenuation introduced to the signal by the cable 15, the level of 
attenuation being frequency dependent. 
Several standards exist for transmitting serial digital video data, which 
vary depending upon the nature of the data being conveyed. For top quality 
professional applications, video data may be conveyed in accordance with 
the CCIR 601 standard, using the 10 bit parallel configuration clocked at 
27 megahertz. Alternatively, D1 data of this type may be transmitted in 
serial form, in which the data is scrambled in accordance with a non 
return to zero algorithm and transmitted at 270 megabits per second. 
The non return to zero algorithm results in a waveform in which a logical 
"1" is transmitted by a signal transition. Theoretically, therefore, the 
highest frequency component which needs to be transmitted is 135 
megahertz, which occurs when two data transitions are conveyed within one 
cycle of the waveform. However, in practice, frequency components higher 
than 135 megahertz may be carried, including components up to 300 
megahertz. 
The energy distribution of a typical serial digital video signal, plotted 
against frequency, is shown in FIG. 2. No appreciable energy is conveyed 
from DC to about ten megahertz and the information of interest is conveyed 
over the range of ten megahertz to 135 megahertz. In the theoretical 
model, no energy should exist at the Nyquist frequency of 135 megahertz 
although, in practice, components do exist at this frequency and beyond. 
A typical frequency response curve for coaxial cable, suitable for 
transmitting serial digital video signals, is shown in FIG. 3, in which 
gain is plotted against a frequency axis, which shares a frequency scale 
with the similar axis of FIG. 2. The cable introduces attenuation, 
therefore the gain is negative. 
It can be seen from FIG. 3 that a simple resistive model of the cable is 
inappropriate because the gain, measured in decibels, varies in proportion 
to the reciprocal of the square root of the applied frequency. Thus, a 
preferred embodiment of the present invention is directed towards 
emulating this response. 
Preferred embodiments of the invention will be configured so as to emulate 
particular types of cable. It is expected that particular embodiments 
will, therefore, be directed towards very specific applications, so as to 
provide accurate results in specific working environments. 
As previously stated, a preferred embodiment is directed towards emulating 
coaxial cable used for transmitting serial digital video signals. Thus, 
the networks will be particularly directed towards the type of cable used 
and the nature of the signals transmitted over the cables. 
A first preferred embodiment is directed towards emulating a 25 meters 
length of Beldon 8281 coaxial cable, used for transmitting serial digital 
video signals. 
As shown in FIG. 4 an input port 31 is serially connected to two resistors 
R2 each of 75 ohms, which are in turn serially connected to resistors R4 
of 5.1 ohms. An inductor L2 of 27.39 nanoHenrys (nH, H times 10 to the 
minus 9) is connected in parallel with resistors R2. A capacitor C1 of 
4.869 picoFarads (pF, F times 10 to the minus 12) bridges the mutual 
connection of resistors R2 to ground. An inductor L4 of 39.21 nH and a 
capacitor C4 of 20.67 pF are both connected in parallel with both serial 
resistors R4. Serial connection of a resistor R3 of 510 ohms, an inductor 
L3 of 16.3 nH and a capacitor C3 of 6.971 pF bridge the mutual connection 
between resistors R4 to ground. 
The network shown in FIG. 4 is mounted within a conductive housing of the 
type shown in FIG. 5. Input terminal 31 is connected to an input BNC 
connector 33 and, similarly, output terminal 32 is connected to a similar 
output BNC connector 34. Thus, the network, which emulates a 25 meter 
length of cable, may be connected in line with existing cable using BNC 
connectors, as commonly used in the art. 
A second preferred embodiment is shown in FIG. 6, in which a plurality of 
networks may be selectively connected in cascade. A first network 41 is 
equivalent to the network shown in FIG. 4 and introduces an attenuation 
equivalent to a 25 meters length of the cable. Similarly, network 42 
introduces an attenuation equivalent to 50 meters, network 43 introduces 
an attenuation equivalent to 100 meters, while network 44 introduces an 
attenuation equivalent to 200 meters of the particular cable under 
consideration. Furthermore, each network may be independently selected by 
the operation of a respective double poled double throw switch. 
A mounting box for the cascade of filters is shown in FIG. 7, in which a 
BNC connector 45 provides an input to the network and an output is 
provided by a similar connector 46. Extending from the box are toggles 47, 
48, 49 and 50 for four double poled double throw switches. Thus, toggle 47 
operates ganged switches 47A and 47B in FIG. 6, thereby introducing 
network 41 to the cascade. Similarly, toggle 48 operates gang switches 48A 
and 48B placing network 42 in series with the input and output connectors 
45 and 46, while toggles 49 and 50 are similarly connected to gang 
switches 49A, 49B and switches 50A, 50B, cascading networks 43 and 44 
respectively. 
Network 42 emulates 50 meters of coaxial cable and has a similar network 
topology to network 41 as shown in FIG. 8. Resistors R6, R7 and 8 are 
similarly configured to resistors R2, R3 and R4 and have values of 43 
ohms, 270 ohms and 9.1 ohms respectively. Capacitors C5, C7 and C8 are 
similarly configured as capacitors C1, C3 and C4 and have values of 11.94 
pF, 42.91 pF and 36.98 pF respectively. Similarly, inductors L6, L7 and L8 
are similarly configured as inductors L2, L3 and L4 and have values of 
67.18 nH, 208 nH and 241.4 nH respectively. 
Network 43 is arranged to introduce an attenuation and group delay 
equivalent to that introduced by 100 meters of coaxial cable. The network 
consists of serially connected balanced resistor pairs R10, R12 and R14 of 
39 ohms, 22 ohms and 7.5 ohms respectively. An inductor L10 is placed in 
parallel with both resistors R10 and has an inductance of 240.2 nH. A 
serially connected resistor R9 of 47 ohms and a capacitor C9 of 42.71 pF 
bridge the mutual connection between resistors R10 to ground. 
An inductor L12 of 13.18 nH and a capacitor C12 of 11.89 pF are connected 
both in parallel with the serially connected resistors R12. A bridge 
between the mutual connection of resistors R12 to ground is formed by a 
resistor R11 of 120 ohms, an inductor L11 of 66.86 nH and a capacitor C11 
of 2.342 pF. 
An inductor L14 of 17.03 nH and a capacitor C14 of 145.5 pF are connected 
in series and said series connection in connected in parallel with 
resistors R14. A capacitor C13 of 3.027 pF and an inductor L13 of 818.2 nH 
are connected in parallel. Said parallel connection is in turn connected 
in series with a resistor R13 of 390 ohms and said series connection 
bridges the mutual connection of resistors R14 to ground. 
Network 44 emulates a 200 meters length of coaxial cable, introducing an 
attenuation of approximately 20 decibel with a group delay equivalent to 
that of the coaxial cable, over the frequency range of interest from 10 
megahertz to 300 megahertz. 
Resistor pairs R16, R18 and R20 are connected in series and have resistance 
values of 43 ohms, 33 ohms and 51 ohms respectively. An inductor L16 of 
110.1 nH is connected in parallel with the serially connected resistors 
R16. A resistor R15 of 43 ohms is connected in series with the capacitor 
C15 of 19.57 pF and said serial connection connects the mutual connection 
of resistors R16 to ground. 
An inductor L18 of 388.2 nH is connected in parallel with resistor pair 
R18. A resistor R17 of 75 ohms is connected in series with a capacitor C17 
of 69.01 pF and this serial combination connects the mutual connection of 
resistors R18 to ground. 
The mutual connection of resistors R20 are connected to ground via a serial 
network consisting of a resistor R19 of 27 ohms, an inductor L19 of 11.58 
nH and a capacitor C19 of 11.35 pF. An inductor L20 of 63.83 nH is 
connected in parallel with resistors R20 and a capacitor C20 of 2.05 pF. 
The network sections are fabricated on a circuit board of FR4 material and 
conductive tracks are laid down using microstrip design principles, to 
give a good transmission bandwidth up to several hundred megahertz. Copper 
tracks are covered with a deposit of solder and good ground planing should 
be achieved during construction. 
All resistors are of the 0805 size of the type configured within a 
resistive layer covering a substrate and providing a tolerance of one 
percent. 
NPO capacitors are employed, with a tolerance of plus or minus one half pF 
for values below ten pF and one percent for those above ten pF, again 
using the 0805 casing size. 
Inductors are individually made by winding enamel coated copper wire around 
toroidal formers, as is known in the art. Data sheets are available which 
specify the number of turns required for a particular inductance, 
whereafter the actual inductance of the inductor is measured using an 
impedance analyser and modifications are made where necessary. 
The completed circuit board is mounted in a conductive housing, preferably 
by soldering a copper braid to the housing, wherein said copper braid has 
previously been soldered to the circuit board itself using high melting 
point solder. The circuit is then encapsulated, so as to hold all of the 
components securely in position. 
A third embodiment is shown in FIG. 9, in which a plurality of network 
sections 91 may be selectively connected in cascade, in response to 
control signals from an automated control processor. 
In the embodiment shown in FIG. 9, 8 network sections are present, arranged 
to emulate cable lengths of 5 meters, 10 meters, 10 meters, 20 meters, 50 
meters, 100 meters, 100 meters and 200 meters. Connection of the network 
sections into the cascade is achieved by a respective relay 92. Each relay 
includes a light emitting diode which is illuminated when its associated 
network section has been connected to the cascade. 
A test signal is applied to an input port 93 and the attenuated signal is 
received at an output port 94. 
The control processor consists of a standard purpose programmable processor 
95, arranged to display output data on a monitor 96 and to receive input 
data from an input device 97. Instructions to connect sections into the 
cascade are supplied to an interface unit 98, which in turn is arranged to 
operate selected relays 92. 
Circuits for the 50 meters, 100 meters and 200 meters sections 91 are 
substantially similar to 42, 43, and 44 shown in FIG. 6, although it is 
preferable to use components with tighter tolerances, allowing extremely 
accurate measurements to be made. 
The 20 meters section uses a circuit topology similar to section 41, with 
suitably modified component values. However, the 5 meters and 10 meters 
sections are difficult to implement using the previously described 
topologies, due to parasitic effects. 
A suitable topology for a 5 meters section is shown in FIG. 10, in which a 
pair of resistors R22, each of 75 ohms, are serially connected to a series 
pair of resistors R24 of 0.8 ohms. An inductor L22 of 10.11 nH is 
connected in parallel with the resistor pair R22 and a capacitor C23 of 
1.798 pF connects the mutual connection between resistors R22 to ground. 
A capacitor C24 of 228 pF is connected in parallel with an inductor L24 of 
22.4 nH, both of which are in parallel with the resistor pairs R24. A 
resistor R25 of 3300 ohms is connected in series with an inductor L25 of 
1280 nH and a capacitor C25 of 3.97 pF. These serially connected 
components connect the mutual connection between resistors R24 to ground. 
In the embodiments described, the network is usable as a hand held test 
device, for use in the field, or as a bench mountable device, for accurate 
testing under laboratory conditions. In an alternative embodiment, a 
switchable device is included as part of another piece of equipment, 
usually operated with the network out of circuit. The network could 
emulate 50 meters of cable, say, and a switch operated to place the 
network in series with the output of the equipment. If, with the network 
in circuit, uncorrupted transmission is still possible, an operator knows 
that sufficient head room is available for the output cable to be extended 
by a further 50 meters. 
Thus, for example, the network could be included as part of equipment used 
for processing digital video signals. It would be included as part of the 
equipment's serial digital output interface and each time the system is 
reconfigured, the network could be brought into circuit to determine 
whether the cable could still be extended by a further 50 meters. 
The preferred circuits described herein are implemented using passive 
networks. This has an advantage, particularly with hand held units, in 
that it is not necessary to provide a power supply. The device can be 
built into a robust casing, without the need for providing manual access. 
However, in alternative embodiments, the network could be configured from 
active circuits, again arranged to emulate the transmission 
characteristics of cable over a frequency range of interest.