In over-the-air (OTA) and cable television distribution systems, television signals may be encoded in analog and/or digital formats. The encoded signals may then be modulated into a channel using digital or analog modulation. Different regions of the world use different transmission standards, which dictate the parameters of transmitted television signals. For example, in North America television signals may be transmitted OTA in the analog NTSC (National Television System Committee) format and/or the digital ATSC (Advanced Television Systems Committee) format. In addition, television signals may be transmitted through cables that connect to a television or associated set-top box. For example, many cable systems use QAM (Quadrature Amplitude Modulation), which includes variants such as 64-QAM, 256-QAM, etc.
In most television distribution systems, the available bandwidth is divided into channels, where each channel is allocated approximately 6-8 MHz of bandwidth. Each channel may include a digital or analog signal (or no signal). FIG. 1 is a graph of a frequency spectrum with channels 101-106. Channels 101 and 105 are analog channels, channels 103 and 106 are digital channels, and channels 102 and 104 have no signal.
Television receivers may include set top boxes, integrated televisions, video capture devices, personal video recorders, and/or other devices such as general purpose computers with software and/or hardware for tuning, demodulating and/or decoding television signals. FIGS. 2 and 3 show conventional arrangements 200 and 300 for demodulating and decoding multiple television formats. In FIG. 2, digital demodulator 220 and analog demodulator 221 receive tuned signals (e.g., intermediate frequency signals and/or digital samples) from separate radio frequency processors 210 and 211, respectively. Radio frequency processors 210 and 211 both generally tune a radio frequency channel (e.g., a band of frequencies such as band 101, 102, 103, etc. in FIG. 1), and may further process the tuned signal (e.g., to produce digital samples of the tuned channel). Demodulators 220 and 221 provide a demodulated signal to signal decoder/image processor 230 for further processing. In FIG. 3, digital demodulator 320 and analog demodulator 321 receive tuned signals from a single radio frequency processor 310.
Television receivers often need to scan a number of radio frequency channels when they are first set up (or when they are reconfigured) to initialize channel parameters. The setup scan may be used, for example, to set up a list of available channels, to pre-determine whether the channel is analog or digital, and/or to pre-determine other characteristics of each channel to enable faster signal acquisition when a user selects a channel to display.
FIG. 4A shows a graph 400 of times to scan a channel with a digital demodulator (e.g., digital demodulator 220 or 320). Time 401 represents an exemplary length of time Tacq_dtv for acquiring a digital television signal (e.g., when the channel does contain a digital signal). Time 402 represents an exemplary length of time Tout_dtv for the digital demodulator to time out when the signal cannot be demodulated (e.g., when the channel contains an analog television signal). Time 403 represents an exemplary length of time Tout_lowpwr_dtv for the digital demodulator to time out when no signal is present.
Similarly, FIG. 4B shows a graph 410 of times to scan a channel with an analog demodulator (e.g., analog demodulator 221 or 321). Time 411 represents an exemplary length of time Tacq_atv for acquiring an analog television signal (e.g., when the channel does contain an analog signal). Time 412 represents an exemplary length of time Tout_atv for the analog demodulator to time out when the signal cannot be demodulated (e.g., when the channel contains a digital television signal). Time 413 represents an exemplary length of time Tout_lowpwr_atv for the digital demodulator to time out when no signal is present.
Conventional receivers generally scan a range of frequencies by attempting to scan each channel using a first mode (e.g., using a digital demodulator) and then using a second mode (e.g., using an analog demodulator) if the first mode fails. FIG. 4C shows a flowchart showing conventional television signal acquisition timing. A receiver may, for example, scan a first channel (e.g., channel 101) during time period 421, a second channel (e.g., channel 102) during time period 422, etc.). For each channel, the receiver first tries to scan the channel digitally and, if the digital scan fails, the receiver tries to scan the channel as an analog channel. Thus, the total time to scan channels 101-106 would be (2*Tacq_dtv)+(2*Tout_dtv)+(2*Tacq_atv)+(2*Tout_lowpwr_dtv)+(2*Tout_lowpwr_atv). Television receiving components are often benchmarked and compared based on channel scan time and stability. Therefore it is desirable to reduce the amount of time spent scanning channels in a mixed-format environment without sacrificing stability.