Analog Television Standards

Vestigial Sideband Modulation (VSB)

All analog television standards use vestigial sideband (VSB) amplitude modulation for the video signal. In standard AM, the modulated signal produces two symmetrical sidebands—each carrying identical information— flanking the carrier. This wastes bandwidth because the same information appears twice. Full single-sideband (SSB) AM would halve the bandwidth but introduces severe distortion at low video frequencies (near DC) because the vestigial rolloff filter cannot have a perfectly sharp cutoff.

VSB is the compromise: the full lower sideband is transmitted (carrying all frequency components from DC to 4.2 MHz), while only a vestige (approximately 0.75 MHz) of the upper sideband remains. The Nyquist slope filter at the receiver compensates for the vestigial rolloff—frequencies near the carrier are attenuated in the vestigial sideband but fully present in the retained sideband, and vice versa. The result is a flat frequency response from DC to 4.2 MHz within a 6 MHz channel, using approximately 4.75 MHz of the channel bandwidth for video (4.2 MHz full sideband + 0.75 MHz vestige + carrier).

NTSC Channel Plan: 6 MHz

The NTSC standard allocates each channel exactly 6 MHz of bandwidth. Within this 6 MHz envelope, the signal components are arranged as follows:

• Video carrier: 1.25 MHz from the lower channel edge
• Lower sideband (full): 4.2 MHz below video carrier → extends to 2.95 MHz above lower edge
• Upper sideband (vestige): 0.75 MHz above video carrier → extends to 2.0 MHz above lower edge
• Audio carrier: 4.5 MHz above video carrier (at 5.75 MHz from lower edge)
• Audio modulation: FM with ±25 kHz peak deviation
• Audio pre-emphasis: 75 µs time constant
• Color subcarrier: 3.579545 MHz above video carrier (QAM modulated)

The 4.5 MHz offset between video and audio carriers is the foundation of the intercarrier sound system: the receiver's IF strip produces both carriers simultaneously, and a 4.5 MHz ceramic filter or tank circuit extracts the audio carrier. This eliminates the need for the local oscillator to track both video and audio precisely—the difference frequency is fixed by the transmitter, not the receiver tuning.

NTSC Sync Timing

The NTSC horizontal sync pulse is a precisely timed negative-going pulse that triggers the receiver's horizontal deflection circuit. The complete horizontal blanking interval contains three segments:

• Front porch: 1.5 µs of blank video level, preceding H-sync. Provides a brief stabilization period before the sync pulse.
• H-sync pulse: 4.7 µs wide, at the sync tip level (0 V). The leading edge of this pulse triggers the horizontal oscillator. The sync pulse contains no video information—it is a pure timing reference.
• Back porch: 4.7 µs of blank video level, following H-sync. The color burst (8–10 cycles of 3.579545 MHz) resides here, centered at approximately 5.3 µs after the leading edge of H-sync. The back porch also serves as the black level reference for the video signal.

The total horizontal blanking interval is approximately 10.9 µs (1.5 + 4.7 + 4.7), leaving 52.6 µs of active video per line. At 15,734.2644 Hz line rate, each line lasts 63.5 µs total. The vertical blanking interval (VBI) spans approximately 20 lines per field (lines 1–20 of each field), during which the beam retraces from bottom to top. VBI lines carry data services: closed captions (line 21), teletext, VITC timecode, and WST (World System Teletext).

PAL Channel Plan: 8 MHz

The PAL standard (used in most of Europe, Asia, and parts of South America) allocates 8 MHz per channel, 33% wider than NTSC. The additional bandwidth accommodates the higher resolution 625-line standard and wider audio provisions:

• Video carrier: 1.25 MHz from lower channel edge
• Video bandwidth: 5.5 MHz (vs. 4.2 MHz for NTSC)
• Audio carriers:
• FM mono: 5.5 MHz above video carrier
• FM stereo: 5.7421875 MHz (M matrix, BTSC-like)
• NICAM digital: 5.8546875 MHz (728 kbps)
• Color subcarrier: 4.43361875 MHz above video carrier
• Line frequency: 15,625 Hz (64 µs per line)
• Field rate: 50 Hz (25 fps frame rate)

NICAM (Near-Instantaneous Companded Audio Multiplex) transmits near-CD- quality stereo audio as a digital signal alongside the analog audio. NICAM uses 14-bit A/D conversion at 32 kHz sampling rate, compressed to 10 bits using quasi-instantaneous companding. The 728 kbps digital stream is transmitted as a DQPSK signal on a subcarrier at 5.85 MHz. This allowed PAL broadcasters to offer stereo sound without sacrificing the existing mono FM audio carrier.

SECAM Channel Plan: 8 MHz

SECAM uses the same 8 MHz channel structure as PAL but replaces the QAM color subcarrier with FM chrominance on alternating lines. The audio carrier spacing is identical to PAL (5.5 MHz FM mono), and SECAM shares the same 625-line/50 Hz scanning standard. The key difference is the chroma encoding: R−Y and B−Y are frequency-modulated onto separate subcarriers and transmitted sequentially, requiring a delay line in the receiver for color reconstruction.

Channel Number to Frequency Mapping

NTSC channel assignments in the US span VHF (channels 2–13 at 54–216 MHz) and UHF (channels 14–83 at 470–890 MHz). The relationship between channel number and frequency is:

VHF Low Band (Channels 2-6):
  f_center = 55.25 + (N-2) × 6 MHz     (video carrier)
  Ch 2:  55.25 MHz  │  Ch 3:  61.25 MHz  │  Ch 4:  67.25 MHz
  Ch 5:  77.25 MHz  │  Ch 6:  83.25 MHz

VHF High Band (Channels 7-13):
  f_center = 175.25 + (N-7) × 6 MHz     (video carrier)
  Ch 7:  175.25 MHz │  Ch 8:  181.25 MHz │  Ch 9:  187.25 MHz
  Ch 10: 193.25 MHz │  Ch 11: 199.25 MHz │  Ch 12: 205.25 MHz
  Ch 13: 211.25 MHz

UHF (Channels 14-83):
  f_center = 471.25 + (N-14) × 6 MHz    (video carrier)
  Ch 14: 471.25 MHz │ Ch 51: 689.25 MHz │ Ch 69: 797.25 MHz

The gap between VHF low and high bands (84–174 MHz) was allocated to other services (FM broadcast at 88–108 MHz, aeronautical, military). This fragmentation forced TV manufacturers to build multi-band tuners with separate oscillator ranges for VHF-low, VHF-high, and UHF.

Analog Audio Standards

Analog television audio varies significantly between NTSC, PAL, and SECAM:

• NTSC (MTS/SAP): The Television Multichannel Sound (MTS) standard adds stereo audio using a pilot tone system similar to FM stereo. A 15.734 kHz pilot (the horizontal scan frequency) indicates stereo presence. The L−R difference signal is DSB-SC modulated on a 31.468 kHz subcarrier (2× pilot). The SAP (Second Audio Program) channel carries a second mono audio program at 5× the pilot frequency (78.67 kHz).
• PAL (A2 stereo): The German A2 system uses a separate FM audio carrier at 5.7421875 MHz for stereo. The L+R sum is on the main 5.5 MHz carrier; the L−R difference is on the 5.74 MHz subcarrier with a 54.6875 kHz pilot tone.
• PAL (NICAM): NICAM 728 provides digital stereo at 728 kbps on a DQPSK subcarrier at 5.85 MHz, offering near-CD quality (14-bit, 32 kHz sampling, companded to 10 bits).
• SECAM: Uses FM audio at 5.5 MHz with a 50 µs pre-emphasis (matching the standard PAL mono audio specification).

The Analog Switch-Off

The transition from analog to digital broadcasting occurred worldwide between 2006 and 2015. The digital dividend freed 700 MHz spectrum (698–862 MHz) previously used for analog TV, reallocating it to mobile broadband. Countries completed the transition at different speeds: Luxembourg was first (2006), the Netherlands followed (2006), the US completed in 2009, and most European countries finished by 2015. The UK completed its analog switch-off on October 24, 2012. The digital transition enabled HD broadcasting, multiple subchannels per frequency, electronic program guides, and interactive services—but analog television's 60-year legacy defined the signal architecture that all modern systems inherited.