High speed data cable with impedance correction
A high speed video cable carries signals according to the High-Definition Multimedia Interface (HDMI) or DisplayPort standards, and includes a raw cable and may include a boost device. The raw cable includes coaxial lines of a characteristic cable impedance higher than the impedance implied in the standards. The correct impedance is observed at the sending end by shunt resistors mounted in the first cable connector. The resultant loss of signal may be made up with the boost device mounted in the connector at the other end of the cable in the case of a long cable.
1. A cable for carrying one or more high speed differential digital data signals and one or more auxiliary signals between a source device and a sink device according to a cable specification, the cable comprising:
a raw cable having one or more dual shielded cable elements, each dual shielded cable element comprising a shield and two shielded conductors for transmitting two polarities of a high speed differential digital data signal, an impedance of at least one dual shielded cable element being higher than a nominal impedance of the cable specified in the cable specification;
a first circuit carrier electrically coupling the source device with the raw cable, the first circuit carrier including a shunt resistor, the shunt resistor being electrically coupled between the shielded conductors of said at least one dual shielded cable element; and
a second circuit carrier electrically coupling the raw cable with the sink device, the second circuit carrier including a boost device for terminating the at least one dual shielded cable element and boosting the high speed differential digital data signal carried by said at least one dual shielded cable element, an output of the boost device being electrically coupled to the sink device.
2. The cable as described in claim 1 , wherein the inverse of a resistance of the shunt resistor is substantially equal to a difference between the inverse of the nominal impedance and the inverse of the impedance of the at least one dual shielded cable element.
3. The cable as described in claim 1 , wherein some or all of said one or more dual shielded cable elements are dual coaxial elements, each comprising two coaxial lines whose shields are joined, each coaxial line enclosing one shielded conductor.
4. The cable as described in claim 3 , wherein the nominal impedance of the cable is 100 ohms, and the impedance of said at least one dual shielded cable element is from about 110 ohms to about 150 ohms, comprised of a sum of characteristic impedances of the two coaxial lines.
5. The cable as described in claim 1 , wherein some or all of said one or more dual shielded cable elements are Shielded Twisted Pairs (STP), each STP comprising a shield enclosing the two shielded conductors.
6. The cable as described in claim 5 , wherein the nominal impedance of the cable is 100 ohms, and a characteristic impedance of a Shielded Twisted Pair is from about 110 ohms to about 150 ohms.
7. The cable as described in claim 6 , wherein the characteristic impedance of the Shielded Twisted Pair is about 150 ohms.
8. The cable as described in claim 1 , wherein the cable specification is the High-Definition Multimedia Interface (HDMI) standard.
9. The cable as described in claim 8 , wherein a characteristic impedance of each coaxial line is about 75 ohms.
10. The cable as described in claim 8 , said one or more high speed differential digital data signals comprise Transition Minimized Differential Signaling (TMDS) signals; and
said one or more auxiliary signals comprise Consumer Electronics Control (CEC), Serial Clock (SCL), Hot Plug Detect (HPD), and +5V Power signals.
11. The cable as described in claim 10 , wherein the boost device comprises an equalizer and an amplifier for equalizing and boosting the TMDS signals respectively.
12. The cable as described in claim 8 , wherein:
the shielded conductors of four dual shielded cable elements extending between the source device and the boost device;
the shielded conductors of the four dual shielded cable elements are adapted to carry respective four high speed differential digital data signals, which are Transition Minimized Differential Signaling (TMDS) signals;
the shielded conductors of one other dual shielded cable element, extending between the source device and the sink device, are adapted to carry an auxiliary signal, which is a HDMI Ethernet and Audio Return Channel (HEAC) differential signal; and
the shields of the four dual shielded cable elements and said one other dual shielded cable element are adapted to carry auxiliary signals, which are Consumer Electronics Control (CEC), Serial Clock (SCL), Serial Data (SDA), Digital Data Channel (DDC)/CEC Ground, and +5V Power signals.
13. The cable as described in claim 1 , wherein the cable specification is the DisplayPort standard.
14. The cable as described in claim 13 , wherein:
the shielded conductors of four dual shielded cable elements extend between the source device and the boost device;
the shielded conductors of the four dual shielded cable elements are adapted to carry respective four high speed differential digital data signals, which are Main Line lanes;
the shielded conductors of one other dual shielded cable element, extending between the source device and the sink device, are adapted to carry an auxiliary signal, which is an Auxiliary Channel (AUX CH) differential signal; and
the shields of the four dual shielded cable elements and said one other dual shielded cable element are adapted to carry auxiliary signals, which are CONFIG1, CONFIG2, Ground, Hot Plug Detect (HPD), and DisplayPort power (DP_PWR) signals.
15. The cable as described in claim 14 , wherein the boost device comprises an equalizer and an amplifier for equalizing and boosting the Main Line lanes respectively.
16. The cable as described in claim 1 , wherein a shield of one of said one or more dual shielded cable elements is adapted to carry an auxiliary signal.
17. The cable as described in claim 16 , wherein the two shielded conductors of one of said one or more dual shielded cable elements are adapted to carry a high speed differential digital data signal.
18. A method for transmitting a high speed differential digital data signal from a source device to a sink device over a digital video cable designed to meet a cable specification, the method comprising:
electrically coupling, in parallel with a shunt resistor, two polarities of the high speed differential digital data signal from the source device to two shielded conductors which are an input of a raw cable, which has an impedance higher than a nominal impedance of the cable specified in the cable specification;
electrically coupling a differential output signal from two shielded conductors which are an output of the raw cable to a boost device whose input matches the impedance of the raw cable;
boosting the differential output signal to generate a boosted differential signal; and
electrically coupling the boosted differential signal to the sink device.
19. The method described in claim 18 , further including transmitting an auxiliary signal from the source device to the sink device over a shield of the raw cable.
20. The method described in claim 19 , wherein the transmitting comprises transmitting over a coaxial cable having a pair of coaxial lines whose shields are joined;
including transmitting the auxiliary signal over the joined shields.
21. The method described in claim 19 , wherein the transmitting comprises transmitting over a Shielded Twisted Pair (STP) comprising a shield enclosing two shielded conductors, including transmitting the auxiliary signal over the shield of the STP, and transmitting the high speed differential digital data signal over the two shielded conductors.