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+ \end{itemize} + \vspace{1cm} + \begin{center} + \includegraphics[width=.9\textwidth]{Figures/systemImpulseResponse.pdf} + \end{center} \end{frame} \begin{frame} - \frametitle{Low Pass IIR Filter} - Very short delay + \frametitle{Echo Impulse Response} + \begin{itemize} + \item With only a forward loop there are a finite number of pulses in the impulse response. + \end{itemize} + \vspace{1cm} + \begin{center} + \includegraphics[width=.9\textwidth]{Figures/echoImpulseResponse.pdf} + \end{center} \end{frame} -\separator{Echo / Flanger / Chorus} +\begin{frame} + \frametitle{Echo Impulse Response} + \begin{itemize} + \item With a feedback loop there will be an infinite number of pulses in the impulse response. + \end{itemize} + \vspace{1cm} + \begin{center} + \includegraphics[width=.9\textwidth]{Figures/echoImpulseResponse50pcRepeat.pdf} + \end{center} +\end{frame} \begin{frame} - \frametitle{Echo} + \frametitle{Echo Impulse Response} \begin{itemize} - \item Delay time is long enough for delayed versions to be perceived as separate sounds. - \item Can have feedback to sustain echo. - \item Can process delayed versions to change its properties, e.g. gain, equalisation etc. + \item If the gain of the feedback loop is too high the impulse response will grow uncontrollably - it is unstable. + \end{itemize} + \vspace{.2cm} + \begin{center} + \includegraphics[width=.9\textwidth]{Figures/echoImpulseResponse100pcRepeat.pdf} + \end{center} +\end{frame} + +\begin{frame} + \frametitle{Impulse Response} + \begin{itemize}\itemsep8pt + \item The impulse response is the time-domain representation of the frequency response. Algorithms can be use to switch between the two representations (Fourier analysis). + \item A system with only forward loops has a {\bf finite} impulse response. + \item A system that includes feedback loops has an {\bf infinite} impulse response, and has the potential to be unstable. \end{itemize} \end{frame} +\separator{Equalisation} + \begin{frame} - \frametitle{Flanger and Chorus} - \begin{itemize} - \item Delay time is short enough for sounds to be perceived as once entity, i.e. deliberate comb filter! - \item Flanger has static delay time. - \item Chorus has a varying delay time, e.g. driven by oscillator. + \frametitle{Digital Equalisers} + \begin{itemize}\itemsep8pt + \item Digital equalisers are constructed using delay networks with very short delay times, and are referred to as filters. + \vspace{.1cm} + \begin{itemize} + \item Feedforward only: {\bf F}inite {\bf I}mpulse {\bf R}esponse filter. + \item Feedforward and feedback: {\bf I}nfinite {\bf I}mpulse {\bf R}esponse filter. + \end{itemize} + \item {\bf FIR} filters generally have more delay paths and have a less ideal response. + \item {\bf IIR} filters can be unstable, and cause more extreme phase changes to the sound. + \item Parametric equalisers (e.g. those in Logic) are {\bf IIR} filters with two feedforward and two feedback loops. \end{itemize} \end{frame} @@ -229,7 +277,7 @@ \begin{itemize} \item Convolution reverberators use {\bf only feedforward} delay networks to simulate room reverberation. \item The impulse response is a representation of the sound in a real or virtual space. - \item Each point in the RIR has a delay, gain and polarity, so very large cascades of {\bf forward} delay loops is needed. + \item Each point in the RIR has a delay, gain and polarity, so very large cascades of {\bf forward} delay loops are needed. \item The calculation process (convolution) has been optimised. It is performed in the frequency domain to improve efficiency but they are still processor intensive. \end{itemize} \end{frame} @@ -249,10 +297,10 @@ \frametitle{HRTFs} \begin{itemize}\itemsep8pt \item HRTF stands for {\bf H}ead {\bf R}elated {\bf T}ransfer {\bf F}unction. - \item It is analogous to an impulse response: + \item It is analogous to a room impulse response: \vspace{0.2cm} \begin{itemize}\itemsep4pt - \item An impulse response describes how the sound waves travel from a source to a microphone within a space. + \item A room impulse response describes how the sound waves travel from a source to a microphone within a space. \item An HRTF describes how the sound travels from the space around you, and into your ears. \end{itemize} \item You can get exceptionally good impressions of space using this approach. @@ -269,26 +317,13 @@ \item Play a test tone through a loudspeaker, e.g. noise or sine-sweep, that contains all frequencies of interest. \item Record the signal at a location within the room. \item Compare the original signal with the recorded signal. + \item ...or the old fashioned way $\rightarrow$ make an impulsive sound and record it! \end{itemize} \item You can do this in any space and them super-impose the measured response onto any future recording. \end{itemize} \end{frame} -\separator{Flanger/Chorus/Phasor} -%\begin{frame} -% \frametitle{Educational multi-track resource} -% -% \begin{itemize}\itemsep12pt -% \item Practice audio files are included in the resources folder for this week. -% -% \item Many more projects can be downloaded from: \href{http://www.cambridge-mt.com/ms-mtk.htm}{\it http://www.cambridge-mt.com/ms-mtk.htm} -% -% \end{itemize} -% -%\end{frame} -% -% \end{document}