22 September 2010—A lot is asked of a phone's display these days. Resolution is getting much higher, and the promise of 3-D viewing is just around the corner. In addition, the display's casing may also house a digital camera, speaker system, and possibly a number of sensors. The unintended consequence is that the cable carrying all the power and data between the display side and the keypad side of a typical clamshell or slider handset is reaching its limit in size and flexibility.
"Current handsets use a bundle of coaxial cables with as many as 40 or 50 wires," says Takehiro Sugita, senior manager in Sony Corp.'s communication systems development section in Tokyo. "As we add more wires, the connectors are becoming too large, the pitch too small, and the cable difficult to bend. Reports of cable damage are increasing."
When a Sony handset designer discussed the problem with Sugita, the designer first asked if a wireless connection would work. Sugita pointed out that unless the display had its own battery, you'd still need a power line. So Sugita instead proposed packing all the display's power and data needs into a single cable. Since then, Sony engineers have built a prototype system that provides both power and 940 megabits per second of bandwidth between the two parts of the phone.
The cable employed is a differential signal transmission cable with twin insulated copper wires and a grounded spiral copper shield. Video, audio, and control data are transmitted bidirectionally using differential signaling along the two wires, while DC power is transmitted along both wires and the shield.
Such disparate data is packed onto one cable by interleaving pieces of each data type, one after the other, and then reassembling the pieces at the receiving end.
In today's handsets, this scheme, called TDD/TDM (time division duplex/time division multiplex), isn't possible because of the data encoder's complex relationship to the system clock.
The Sony technology uses a master-and-slave set of two mixed-signal chips. The master chip resides in the main handset portion, along with the clocks controlling the display and camera, while the slave chip resides in the display casing. Sugita explains that the master chip uses the signal from the handset's clock to generate its own system clock, which is suitable for transmitting different data signals across the single cable interface to the slave portion using Sony's own TDD/TDM scheme.
A second key technology helps Sony pack in the bits. Using conversion tables, its multilevel encoding technology encodes bits of data at five voltage levels using what's called the quinary (base 5: 0 to 4) numeral system, binary data to quinary data using conversion tables. This enables a data rate of 940 Mb/s when encoding takes place at a frequency of 540 megahertz—roughly a 75 percent improvement over standard encoding systems, such as non–return to zero (NRZ) or 8b/10b encoding.
"Multilayer encoding based on the quinary system would normally mean the data conversion table would be very large, so the size of the chip would have to be large," says Sugita. "Instead, we use a combination of small tables, which is more manageable and reduces the chip size."
A high-pass filter separates the data signals from the DC power signal. The resulting direct current and low frequency DC waveform would suffer error-inducing distortions, "but we are able to maintain DC balance by selecting DC-balanced values from the conversion tables, so no errors occur," says Sugita.
Sony is keen to use the interface technology in its products as early as possible. To that end, it teamed up last November with Kyoto-based Rohm Co., a major manufacturer of custom ICs, to speed things along. The two companies have since jointly developed the analog portions of the test chips.
Sony will license the intellectual property for the digital portion of the new technology to Rohm; the goal is to produce a commercial product quickly. Rohm would not comment on when this product might be available, but the company is reportedly in the midst of finalizing specifications with customers, whereupon it will begin manufacturing.
About the Author
John Boyd is a Tokyo-based technology and science journalist. In April 2010 he described another Sony advance: wireless chip-to-chip communication.