Wideband multi-channel quadrature amplitude modulation of cable television signals

A multi-channel direct-to-RF QAM modulator for modulating a plurality of digital data streams onto a multi-RF output, comprising:

• QAM encoding means for encoding the digital data streams into a like plurality of QAM-encoded symbol streams;
• IFFT processing means having each QAM-encoded symbol stream applied to a specific complex frequency input thereof, said IFFT processing means producing a time-domain signal representative of the plurality of QAM-encoded symbol streams modulated onto a set of uniformly spaced carrier frequencies in an intermediate frequency IF band;
• post-IFFT filtering means, producing a filtered time domain signal, for performing the combined equivalent of baseband filtering, anti-imaging filtering and rate interpolation to compensate for a difference between a QAM symbol rate and a channel spacing;
• digital-to-analog conversion means for converting the filtered time-domain signal from complex digital to complex analog form to produce an analog multi-channel IF stream;
• synchronously-clocked quadrature modulation means to frequency-shift the analog multi-channel IF stream upward by one-half of the master clock frequency onto said single multi-RF output;
• and
• digital quadrature correction means between the post-IFFT filtering means and the digital-to-analog conversion means for compensating quadrature error in the synchronously clocked quadrature modulation means.

A method for direct-to-RF multi-channel QAM modulation of a plurality of digital data streams onto a multi-RF output, comprising:

• providing a high-frequency master clock source at a master clock frequency;
• providing a plurality of digital data input streams;
• encoding each of the digital data streams into a set of QAM encoded streams;
• processing the QAM-encoded streams at the master clock frequency to modulate the plurality of QAM-encoded streams into a single complex digital multi-channel IF stream encoding the multiple QAM encoded streams onto a set of uniformly spaced carrier frequencies in an intermediate frequency band, said processing comprising inverse FFT (IFFT) processing and post-IFFT filtering, said filtering comprising a combined baseband and anti-imaging filtering;
• converting the digital multi-channel IF stream to analog form;
• synchronously frequency-shifting the analog multi-channel IF stream upward by one-half the master-clock frequency onto said multi-RF output;
• and providing digital quadrature correction between the post-IFFT filtering and the analog converting for compensating quadrature error in the synchronously frequency-shifted analog multi-channel IF stream.

A wideband multi-channel direct-to-RF QAM modulator for modulating a plurality of digital data streams onto a single multi-RF output, comprising:

• a high-frequency master clock source operating at a master clock frequency;
• encoding means for encoding each of the digital data streams into a set of QAM encoded-streams;
• processing means for simultaneously converting the plurality of QAM-encoded streams into a single digital multi-channel IF stream encoding the multiple QAM encoded streams onto a set of uniformly spaced carrier frequencies in an intermediate frequency band, said processing means including an inverse FFT (IFFT) and a post-IFFT combined filtering means, said combined filtering means performing a combined equivalent of baseband and anti-imaging filtering;
• digital-to-analog conversion means for converting the single digital multi-channel IF stream into an analog multi-channel IF stream;
• synchronously-clocked quadrature modulation means to frequency-shift the analog multi-channel IF stream upward by one-half of the master clock frequency onto said single multi-RF output;
• and
• digital quadrature correction means for compensating quadrature error to precompensate for subsequent DC offset in the synchronously clocked quadrature modulation means.

A multi-channel direct-to-RF QAM modulator for modulating a plurality of digital data streams onto a multi-RF output, comprising:

• QAM encoding means for encoding the digital data streams into a like plurality of QAM- encoded symbol streams;
• IFFT processing means having each QAM-encoded symbol stream applied to a specific complex frequency input thereof, said IFFT processing means producing a time-domain signal representative of the plurality of QAM-encoded symbol streams modulated onto a set of uniformly spaced carrier frequencies in an intermediate frequency IF band;
• post-IFFT filtering means, producing a filtered time domain signal, for performing the combined equivalent of baseband filtering, anti-imaging filtering and rate interpolation to compensate for a difference between a QAM symbol rate and a channel spacing;
• digital-to-analog conversion means for converting the filtered time-domain signal from complex digital to complex analog form to produce an analog multi-channel IF stream;
• synchronously-clocked quadrature modulation means to frequency-shift the analog multi-channel IF stream upward by one-half of the master clock frequency onto said single multi-RF output;
• and
• digital quadrature correction means for compensating quadrature error to precompensate for subsequent DC offset in the synchronously clocked quadrature modulation means.

A method for direct-to-RF multi-channel QAM modulation of a plurality of digital data streams onto a multi-RF output, comprising:

• providing a high-frequency master clock source at a master clock frequency;
• providing a plurality of digital data input streams;
• encoding each of the digital data streams into a set of QAM encoded streams;
• processing the QAM-encoded streams at the master clock frequency to modulate the plurality of QAM-encoded streams into a single complex digital multi-channel IF stream encoding the multiple QAM encoded streams onto a set of uniformly spaced carrier frequencies in an intermediate frequency band, said processing comprising inverse FFT (IFFT) processing and post-IFFT filtering, said filtering comprising a combined baseband and anti-imaging filtering;
• converting the digital multi-channel IF stream to analog form;
• synchronously frequency-shifting the analog multi-channel IF stream upward by one-half the master-clock frequency onto said multi-RF output;
• and
• providing digital quadrature correction for compensating quadrature error to precompensate for subsequent DC offset during the synchronously frequency-shifting step.